I will probably be told that this is all wrong, but in the past I've used creased paper tape, and used a sponge (just stole one from under the sink) to apply watery PVA to the joint area, then laid the tape, then used the spong to squeegee it down and remove all bubbles, as well as wiping away excess PVA. Then the filler goes on top once it is dry.
I'm sure somebody will be along in a minute who doesn't just make these things up as they go along