ALWAYS add nutes to water and stir well before offering it to the ladies. Adding it to the soil will cause burn spots (too concentrated) and voids. (no nutes at all)

If I only have a few plants, or for those in veg where I won't go through an entire gallon...I either give the rest to my houseplants, or mix half a batch. If the instructions are for 1 tablespoon per gallon, mix half a tablespoon (1.5 tsp) in half a gallon.

I adjust my water's ph to about 6.8 or 6.9 before adding the nutrients. Once the nutrients are in and mixed, it brings the ph down to about 6.4 or so. Perfect for the ladies.
(ph your water at the recommended temp ranges, or test results will vary a tad)

Whenever I water or add nutes to my 5 gallon pots, I'll add a half gallon or so to each pot. Then, I'll go back to the first pot, and slowly add more till I get a slight run-off. Just a minor trickle. (more will ooze-out over time) And I continue down the line till they are all bloated and happy.
This allows the medium to absorb the water and nutes, instead of finding the quickest route through your soil, and not absorbing enough. I use the same method with every size pot I grow in. The 5'er was just an example.