Put the cut edge against the return wall (internal corner) or up against a door lining.
You can cut a chamfer with a stanley knife or get a special plane if you have to join square ends.
Buy your boards long enough to go floor to ceiling then you will only have vertical joints to do.
Use regularized timber or CLS as it will give a more even surface to screw the boards to.
Fibre glass tape is a lot easier to use than the paper tape, Gyproc easyfill is what I use to fill with, get a proper mud pan, 8" taping knife and drywall sander.
Some useful toolsJason