I don't pay much attention to the timing between feedings and doing water changes.
I guess I'd wait an hour or so after feeding, but I'm not that energetic to do early morning water changes. lol
You should have a clean never seen soap bucket or container to do your water changes. I have one for dirty water and one for new water. Do you have a gravel vacuum? You'll need one to get out the left over food and fish waste.
Dechlorinators usually work immediately. Pour the new water into the tank slowly. You don't want to toss the fish around, stir up gunk that might be left on the bottom or displace gravel/decorations. Be sure the new water is close to the same temp as the tank water.
Good luck, you'll do fine.
