Taking a look at the diagram (67 to 68 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. 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.
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 which is not shown on this Diagram so your car even registered as a 1969 or even 1970 may not have a Starter relay, yet conversly 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 includes the relay. The relay is on the inner fender on the Right side (from the drivers seat) Look at the graphic in the Diagram to get more information, ie. the number of connections and colours 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 cicuit.

From here the Brown wires denoted also by the "N" distribute the voltage to other parts of the system. 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 peice on charging systems later). But even with the engine stopped there is still full voltage in these wires. A third Brown wire is also connected to the Fuse box. From here the voltage is distributed directly to the Headlights, then to a line branching of to the Headlight Flasher and Radio on its way to the Ignition Switch. This connection is all on one side of the fuse Box. So the voltage does not pass though a fuse to the other components except as you can the the Radio with its own in-line fuse, but the Headlights and everything else listed are not fused.
The Brown wire is the next level down in voltage suppy for the system after the Black battery cable. It will carry around 50 amps or so. Again be very careful around brown wires.

The Brown wire is suppling voltage to the Ignition Switch at Terminal 3 when the Switch is in the OFF position Terminal 1 and 2 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 2 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.
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 2 but now also to terminal 1 to the Red and White "RW" wire which gives Voltage to the Solenoid which in turn engages the starter. When you release the ignition switch the voltage is cut to the Terminal 1 so disconnecting the Starter. But Terminal 2 maintains voltage whether in first or second position. When the Ignition is turned OFF the voltage is disconected at Terminal 2 hence the Engine stops.

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. The easy way to see if the voltage is getting to the Solenoid and in turn to the fuse box, is to turn on the lights. If the lights don't work you check the connections on the solenoid are OK assuming the Battery is not flat.

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 Diagaram 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 Soleniod. 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 daigram you can see where the voltage is suppost 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 Soleniod 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. 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.
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 ciruit to check if something is working.
But you need an understanding of voltage flow as it is affected by bad grounds, incomplete ciruits 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