Just go by the directions, adding however many teaspoons per gallon it says to use for whatever phase of growth your plants are in... Just mix it right in with your nutrient water.
It'll differ from phase to phase, i.e. when the plants are seedlings it'll say to use this much, then when they're midsize vegetative you'll use a little more, mature/transitional use a little more, etc.
It's never a large amount used at a time, usually 2-3 tsp per gallon of water
A gallon will last you a LONG time.
Quarts last me 2 or 3 grows (I grow 2-3 plants at a time).