How is it done? Say I want to have a particular NPC wear a particular armor or item? I found the command to remove stuff, but not to add/change specific items.

I have always reversed pick-pocket the NPC with the new armor. Be careful, once you give them a new item, the only way to change it is to give them a better item. For example: I gave a female NPC a set of combat armor and didn't like the look and tried to change it to a T6 China Dress. Since the dr on the armor was higher than the dress, the change did not work. It also works the same way for the item's condition. You cannot change a 100% armor with anything less.

Wouldn't removing all that NPCs equipment with console commands and then reverse pickpocketing again solve that problem?

You are absolutely correct. I didn't think of that. Thank you. I will do that in the future.