We are preparing to tape and mud the 'inside corners' of a diagonal attic ceiling, where the short walls meet the ceiling. What is the best way to achieve a straight line? Is there a special tool we can use? Free-handing is not working for us. Our 90 degree trowel works fine for vertical inside corners, but of course the 'corners' we are trying to do are horizontal.
I've had decent luck not taping inside corners at all. Instead I've use a sanded caulk. The key is to have a nice sharp edge to work with where the two pieces of sheetrock meet.
But if you want to mud and tape, there are tools for that:
They come in various angles, but most can adjust the angle merely by angling the handle yourself.