i had this same question and asked it a couple days ago. i also asked someone at an indoor growing store.
there are a few ways of doing it.
1. mix the nutrients in with the water and water the plant(s). this can be liquid nutes or solid soluble (dissolve in water) nutes
2. if you get soil that has no nutrients in it you can add your own (or you can add more to soil as needed with nutes in it) by taking the nutrient powder and putting a certain amount in on the top (not sure how much you're supposed to put in) and mixing it in with your hands.
I measure it out with a tablespoon and put that in a 1 gallon jug and shake it up really well then water. I'm using Earth Juice grow and FF Big Bloom every other watering with fresh water in between.
once my plants get bigger, i usually only feed them once a week. Full strength doses. But i start with real good soil. And am growing in containers...
and if you read this jeffca, good job with the answer.