If you're doing a tag search like morutarou while logged in, you should see a combo box under the search box, labelled "Mode". Change it from "View" to "Tag script" and another text box will appear. Any tags you enter here will be added, any prefixed with a minus sign will be removed. Then click on each post you wish to edit, queuing up as many as you like. Make sure you switch back to "View" afterwards so you don't keep editing by accident, as it gets saved between searches.
Be very, very careful doing this, as it's easy to mess up. If you do, click on Changes in the blue bar at the top and hit "Undo" on each edit you wish to revert. Each edit is labelled with a username so it's easy to find which ones are yours.
Happy mass-tagging!
Ahhahhahhahha
It did happy poo poo toooo
I can only laugh~It's nothingAhh, don't worryDon't they realize they did their things?
Though they could recognize them now, but it's pointless now...Chomp chomp!Ah, don't mind it and eat your food...
Mister just had some dizziness...Hm? Mishter?
What ish it noje?A-are you okay nanoje?