Use an estimate first, if you are running CO2, you can maintain the tanks in high light level and then
1)use this formula, 1.5W LED per gallon. There are other ways to estimate but each method have their drawbacks and advantages.
2)Choose color of LEDs
3)Determine whether you want controller, dimmability or just on off
4)Look for the right fixture(not just a heat sink)-you need splash guard!
5)Ask yourself whether you can solder, the most important part in soldering is to beware of wire frays.
6)Watch couple Youtube tutorials of other peoples' builds but unfortunately MOST are done incorrectly including couple videos from a DIY LED store(LOL) Anyway, you can still learn from other people's mistakes.