You don’t need to scratch the bonding before applying the finishing skim, give everything 2 very good coats of PVA, the second after the first has dry (I always apply the 1st coat the night before) & the 2nd just before plastering. I don’t use a corner trowel & have my own method of getting the corners nice & straight; to be honest, I think using one may be more hindrance that help to a beginner. If you’re not confident about the result, I would use a stop bead but if the wall is already uneven, it may be of limited help. I always use a stop bead on window reveals against the frame as they are fiddly & it guarantees you a nice straight edge where it’s very noticeable.
Plaster one wall at a time, don’t attempt to work a wet corner; & do a wall that you won’t see much of first to see how you get on. Good luck, your gonna need it but you never know, you might be a ‘natural’!