Taking a look at the diagram (68 to 70 MGB) you can see the mechanical parts of the circuit are shown by basic graphics and named. Then the coloured lines represent the wires used to connect them. It is just part of the complete diagram as we are at the moment only interested in the starting section of the diagram.

The graphics of the parts are as if you are looking at the connections, as the Starter shows the Solenoid mounted on the Starter motor with the connections in the correct position. Same for the Regulator, Alternator and Ignition Switch and Starter relay. They also give the connection numbers as well. (Though these are not always easy to see in real life).
Looking at the graphic representing the Batteries you can see it shows there are two of them, denoting a car when two 6 volt are used and connected in Series to supply 12V to the system, shown by the way the POS and NEG are marked and connected. The heavy black line representing the Battery cables. The sign on the end of the NEG terminal denotes a ground to the Unibody or frame. Though in the case of the Alternator it shows grounding though the Body of the Alternator not with an external wire.
In British publications a Diagram is called a "Schematic" and a Ground is referred to as "Earth"

In the period around 1968 was when a Starter Relay was introduced shown on this Diagram but even if your car is registered as a 1969 or even 1970 may not have a Starter relay, yet conversely a 1968 may have one fitted. So make sure you are on the right Diagram, click HERE to change to the Diagram and notes that does not have the relay. The relay will be on the inner fender on the Right side (from the drivers seat) Look at the graphic in the Diagram to get more information, as in the number of connections and colors of the wires connecting to the Relay. This information will help you find it, assuming you have one.

Looking at the Diagram the Black battery cable connects to the Starter Solenoid this provides voltage for the Starter when called for, but also as a distribution point for the electrical system. The Amperage can be very high at this point as the starter requires 100 to 180 amps to work though it depends on the condition of the starter itself as well as the engine compression. Suffice to say be careful around this connection, disconnect the battery if you are removing the starter or any of the wires related to it. This is not a fused circuit.

From here the connection on the starter supplies the Alternator and Regulator. Once the Engine starts the flow of Voltage is reversed on these connections with voltage to the battery. (I will try and do a piece on charging systems later). But even with the engine stopped there is still full voltage in these wires.
A third Brown wire "N" from the Starter goes to a distribution point which then connects using a Brown "N" wire the Fuse box, Headlight Switch and the Ignition Switch. From the connection on the Fuse Box itself Voltage is supplied again using the Brown "N" wire to the Starter relay.
This connection is on one side of the fuse Box. So the voltage only passes though the fuse to the Purple "P" wires used for the Horns, Headlight flasher and the Courtesy lights. All the Brown wires are not on a fused circuit only the Purple that have passed though the Fuse box are fused. So be very careful around the brown wires and connections. The Brown wire is the next level down in voltage supply for the system after the Black battery cable. It will carry around 50 amps or so. Again be very careful around brown wires.

The terminals on the later Ignition switch has more connections (seven in all) and they are numbered differently. Note that only one extra is required. The number "3" connection is now to give voltage to the Radio and the Cigar lighter via the White/Green "WG" when the Ignition is in the "Accessories" position.
The Brown wire "N" is supplying voltage to the Ignition Switch at Terminal 1 when the Switch is in the OFF position Terminal 6 and 7 are dead with no voltage. (Or one would hope so). When you turn the Car Ignition ON moving the Switch to the first position the Terminal 7 with the White wires "W" will become live. Voltage will flow to the Alternator though the Ignition light in the dash. To the Fuel pump which you should hear ticking a couple of times. Also to a few other things that we are not dealing with here. So the White wire is the next level down in voltage supply to the system. It also supplies the Fuse box but again not a fused circuit. Also terminal "3" will remain live for the Radio etc.
On the Fuse box on the other side to the Brown and White wires are the Purple "P" and Green "G" wires. This is the fused side of the system, purple tends to be the Horns and the Green the accessories.
When you turn the Ignition to the second position (Starting) the voltage is still supplied to Terminal 7 but now also to terminal 6 to the Red and White "RW" wire which gives Voltage to the Starter Relay which in turn sends voltage to the starter. When you release the ignition switch the voltage is cut to the Terminal 6 so disconnecting the Starter. But Terminal 7 maintains voltage whether in first or second position. When the Ignition is turned OFF the voltage is disconnected at Terminal 7 hence the Engine stops.
Think of the Starter Relay simply as a switch that is turned on and off by passing voltage though it. The terminal C1 Brown "N" wire has voltage from the fuse box. It is supplied from the Live side of the fuse box so is not on a fused circuit, and always has voltage.
When voltage from the Ignition switch is sent from terminal "6" Red/White wire "RW" to the Relay terminal W1. The Relay in turn sends voltage to the Starter Solenoid from terminal C2 Brown/White "WN" wire which cranks the starter. You can hear and feel the Relay click when it is working. The wire from the Terminal W2 to ground is important if the relay is not working check the ground is connected.
The purpose of the relay is to reduce the amperage passing though the Ignition switch, remember this later switch has more connections so I suspect the internal contacts for the switch are smaller hence the need for a Relay.

Before you start checking the wiring make sure the battery is Ok and showing 13 volt with a voltmeter and the connections to your cables are good. Don't assume check them. Take them off and clean them. Check if you have voltage at the Fuse box at fuse 7-8 you can tell as it has the Brown "N" wire on one side and the Purple "P" on the other. If that's Ok check the Relay for working. Voltage at C1 should be present all the time, then C2 and W1 when the Ignition switch is in the "Starting" (second) position.

In the Video I put a temp fuse 20 amp on the Battery connection, now hopefully you understand why. The engine will not crank with the fuse in place, it requires too much amperage and will blow the fuse. If you put a fuse big enough to manage to crank the engine in there then you are missing the point.

Now using the Diagram in finding a fault makes life a lot simpler. If the engine does not crank, (that is turning over)
Using a 12 volt Test light connect your cable to ground, that is any handy piece of metal on the car that is not painted. The Alternator casing is usually good. Then qualify the light by putting the pointer onto a voltage source. (Each time you get a no voltage when expecting a voltage to be present check the test light is working). A good voltage source is in fact the very thing you are testing, the black wire on the Solenoid. This is straight from the battery or batteries if it is dead here then go to the battery connections. After making sure the ground is OK on your test light, waggle it around or move it somewhere else. The battery connections include the Ground strap connection, note on the MGB the ground connection is in the worst place it could be. Make sure it is grounding on the bolted connection to the Body.

From the diagram you can see where the voltage is suppose to be going. If when you turn the Ignition to ON, (the first position). The Ignition light does not come on, as already suggested turn on the lights. If they don't work look at the connection right on the Solenoid where the brown wire is connected to the Black cable. If it is ok and voltage is showing in the Brown wire move to the fuse box where it is connected to the fuse box see if your voltage is present there. If not your problem is in that section of the harness or the Distribution point. If the lights work then check the voltage at the Ignition light holder behind the dash. The bulb is more than likely blown. If not the Regulator in the Alternator could be at fault, to test simply disconnect and ground the Brown and White wire at the Alternator and see if the light comes on.
If you have Ignition and no cranking then follow the voltage though the Starter Relay, you can trigger the Starter relay by putting a jump lead with voltage to the terminal W1 Red/White "RW" wire, again make sure W2 black "B" wire is a good ground. You can check the ground by a jump lead from W2 to the ground clip on your test light. (As the test light works it is a proven ground) then try again voltage to W1 with the ground jump lead in place. As you can see it makes life a lot easier if you follow the diagram. I am not going to try and explain every step and fault that can be present, the idea here is to give you a working idea of how to check for problems using the diagram. By using temporary grounds and Temp power source you can isolate parts of the circuit to check if something is working.
But you need an understanding of voltage flow as it is affected by bad grounds, incomplete circuits and of course no voltage. Remember all the circuits talked about here are not fused. Use a fuse on the battery as in the video. If everything suddenly stops working check the temporary battery fuse.

For the complete Wiring Diagrams for MGB click HERE