Well the first key is planning. You need to know what you want to add to the car. Install a second fuse panel so that you only need to run a 10 gauge wire from the starter solenoid (and fuse it as close to the solenoid as you can) to feed anything you want to run in the car without taxing any factory wiring that was not intended to run something high draw. You also must know that less is more. Adding wires to a car is always tough to do well. Add them to looms when ever you can and this is curtail when adding relays. In the case of the under dash lights, they draw so little power that the amp clip cant even read the current. The are available on ebay as LED strips and come in MANY sizes. The one I added in the white car is 60 inches. For the blue pinto, since it has a console, i bought two 24" lengths. Here is the switch panel for the blue car:
My trick for multiple low current leads to minimize wiring.
The middle tabs are for the switch and go to, in this case, under dash lights, fog lights and blue parking lights
Here is the fuse panel right when I installed it. Note the large red wire above and the smaller lead for the radio. This was in the blue pinto right when I installed it. The power lead is hidden now but the smaller ones are tucked away but due to geometry are still visible from under the dash.
On the white car (the one with the blue lights shown) could not add my usual switch bank to the under dash since I went to GREAT trouble to make the dash look like an early car. I added the switch to the inside of the center console.
and hid the wires under the carpet
Take your time wiring. Don't use a 12 gauge if 22 will do. Solder where ever you can and relays and fuses are your friends!