Pre mix is a powder that you mix in with your light mix, it's rich and the plants feed off that, you just feed water, maybe a top dress midway through, but you have to be on your toes, and probably not the best way to try and cut your teeth, so I would recommend using Fish Mix, and Bloom, tried and tested for years
Start with fish mix at 1ml per liter, and build up as the plant gets bigger peaking around 3-4ml depending on plant size, then when it autoflowers overlap the fishmix with the introduction of bloom, then slowly up bloom, and lower the fish mix. Stop using fish mix around day 40ish
there is a great thread, with loads of info, a few feeding scedules by various BioBizz users
loads of questions and myths busted in this thread
And yes, always feed, you can always back off, if you think your over feeding, but never feed plain water until the end
And don't water to run off, keep all the liquid in the pot
I hope this helps ya