For me I use a multimeter that has mA setting. First you want to remove negative battery cable. The put the multimeter b/t negative cable and neg terminal on battery. The setting the multimeter should be on first is 10A, unless your multimeter has a lower setting. If you start off with the multimeter in mA you will pop the internal fuse of the multimeter(MM).
Once of 10a setting see what your reading is once setup. This is with key off also BTW. If it shows like 1 or .5 or something like that. Then something is draining it quite a bit.
Start pulling fuses (take note as to where they go) until you all of a sudden see the amps drop. Now change the multimeter to 200 mA or the highest setting for mA. Check and see what it is, it should be lower than 100mA on most cars. Depending on options and accessories your car came with. A manual should tell you what the key off draw should be.
Anyways once you found the problem, look up the circuit wiring diagram and see what all it powers. Usually it's a radio or door switch that give the most issues.