[automaton 3]

I've done boatloads of this work. Lightly sand around the joints on the painted surfaces first to promote adhesion. Don't use hot mud, it sets up fast and is a pain to sand. The general purpose mud is easier to work with. If you buy it pre-mixed it you'll need to thin it down slightly.

The paper tape has to have a thin layer of mud behind it before application, then another thin layer over top as your starter coat. In our dry climate I tend to apply the tape wet or pre-coated with mud to promote good adhesion. Then add thin layers and feather the edges (you should apply differential pressure on the knife to feather- some really good youtube videos on this).

Keep in mind the goal is to create a very gradual hill over the seam so it become imperceptible, and not to try to make it flat. Do not over-sand. Get a very bright light and hold it cross-ways to your seams. Anything noticeable will show up after paint.

Another tip. If you happen to get bubbles or lifting in the paper it won't disappear with more layers of mud, that part of the tape probably has to be cut out and re-done.