You can use a painting pad, but those will follow the line of the ceiling so if it is not straight, your paint line will not be straight. One way to get above stairs is to put the extension ladder at the bottom of the stairs and then run a plank from that ladder to another ladder at the top of the stairs which is usually a step ladder. You can also use a brush clip mounted on an extension pole, but that is very hard to control to make straight cuts. Many times though above stairs there is only about 4-6 feet that I can't reach so the brush clip works for cutting in only that small section.

I just did this with my husband. We have stairs that curve around a wall in our two-story foyer. At the top of the wall over the staircase, we put the extension ladder on one step and he cut in at the top of the wall. The rest of the foyer we painted with the help of a scaffold we rented from Home Depot.

Scaffold (in one form or another) is often required to paint stairwells.
A fold up type ladder can be placed on the steps, the 2x lumber used to make a solid base and a short extension ladder used to reach the top of the wall and ceiling.

I use heavy C-clamps to hold the 2x lumber to the fold up ladder, and also add a 1x2 to provide a 'catch' or the ladder so it cannot slide off the 2x lumber.

If you place the multi fold ladder as far away from the wall to be painted as possible you can typically obtain a safe angle for a short extension ladder.