When ever I add water to the plants medium (after a certain age) I always add nutrients to said water before giving it to them. When I was feeding synthetic liquid nutes to my plants it would give me an option. Cuttings/seedlings, early veg, late veg, early flower, late flower. For what ever stage of life your plant is at I would follow their directions. Say I was in late veg, and it said 5 ml of grow, 8ml micro, and 2 ml bloom (because they were a three part system) per litre of water for late veg. I’d fill up a gallon of water in a watering container. A gallon is roughly 4 litres without getting technical. So 4 x 5ml is 20 ml of grow I would add into a gallon of water. So on and so forth. Then since they’re being fed liquid nutes, giving them a flush to rid all of the salts around the roots before going into flower and before harvest is always recommended