you on about the 2 opposing edges / sides of the angle on the ceiling or the 'gable' end where you got a triangle to work into?
like spunky says, either fashion yourself a nice little pointy trowel from an old skimming trowel, or use a leaf and square / trowel and square small tool...
or if its the ceiling, catch it early and use the tip of the trowel horizontally as you'll catch your knuckles trying to pull it at 90 degrees or bond it out and put a roll in...