Beginner has just got his growbag and three flowerpots with the bottoms cut
off sunk into it.

What is the easiest and most troublefree way to feed the tomatoe plants that
are going in there?

My thinking is to get a couple of handfuls of that granular slow release
'growmore' in top of the flowerpot and water that in, when i water the
plants. Would that be an ok thing to do?