Switch multimeter to measure DC volts. Closest setting larger than 12 (Usually 20 or thereabouts)
Make sure the black lead is plugged to black socket on multimeter
Red lead plugged to dc volt red socket. (Check multimeter manual if not sure)

Go to battery. Put black lead/probe on -ve terminal, red on +ve terminal. Should then measure 12 or thereabouts
Now put black lean/probe on the body somewhere (Try where the battery -ve connects to the body) then put red on +ve at battery. Should have same voltage as previously (give or take). If no voltage then battery is not good or connections not good !

Now go to front of car.
Place black probe on one of the engine bolt/studs, then place the red on one of the fuse terminals. (Always good practice to earth first with black, then apply red) If they are live then you will get 12volts or thereabouts. Start by checking the brown wire connection. Then move the red probe to the other side of the fuse, should be the same voltage.
Repeat for the other side. Note that this side is only live if ignition is on.
White is the feed or "delivery" side, Green (Mostly) is downstream supply to things that have power when ignition is on.

Now your marching ! (As they said on Stripes - Look it up !)

Repeat the process using the electrical diagram as a guide, working back to your ignition switch and the wiring to the clocks (Speedo etc).

Hope that helps.
Look up Youtube for auto electrical. Should be lots of good how to's.