Very carefully. A tap is hard, brittle steel. Even a large one will snap off if twisted too hard. Use any kind of oil on the threads and be sure the tap is run in perfectly straight. Twist it in a quarter turn or so, back out, clean, oil, twist in another quarter to half a turn, repeat, repeat, repeat. If it jams back it out very carefully, don't break it off. Don't muscle it. Be sure you have the same size tap as the threaded hole are.Might need to run a tap in each hole.
You can make your own chaser by using an angle grinder or dremel to put some notches in a regular bolt. I’ve done that in the past when I had some gummed up threadsI can’t find a thread chaser in that size anywhere.