How do I use a car fuse light tester

Here are a series of good videos on the subject,

https://www.youtube.com/watch?v=gKgmIcmp7eo

Connect alligator clip to ground, turn ignition switch to "ON," and touch the probe to each test point on top of the fuse. The light should come on when you touch each test point--if both sides don't light up, the fuse is bad. There is no need to remove the fuse from the fuse box, which allows you to test all under dash and under hood fuses in several minutes.

Make sure you look at what systems are protected by each fuse. Follow the guide inside the fuse box cover or the owner's manual (if you still have it) since some circuits need to be activated in order to have power applied to that fuse.
Also, some accessories are protected by circuit breakers instead of fuses.
Are you having any symptoms or systems that don't work?