Painting a stairwell can be a difficult and dangerous task if you have a very high ceiling. In most cases, you will need to use a folding ladder and climb to reach the top. If you are not sturdy on a ladder, you should hire someone to do it for you. However, if you can handle the ladder, you can get this project done in a day. If you have a little time and patience and lots of organisation, you'll have no problem completing this job.
Place painters tape along the stairwell trim and anything else you do not want to get paint on. Cover the stairs with newspaper or something that won't slip, such as a painter's tarp, under the ladder. Use a ladder that folds at 1-foot intervals. This will allow you to set it up on a higher and lower step.
Cut in the bottom of the walls on both sides along the stairs, three or four stairs up. Cutting in is the process of painting from the edge of the trim to an inch or two away with a brush so your roller does not touch the trim or leave gaps in the paint where the roller won't reach. Next, set up your ladder so you can reach the top of the same area. Cut in along the ceiling. Keep rags handy in case you should drip the paint on the trim or on your ladder. If you wipe it right away, it will come off.
Move the ladder up and roll the paint on between the area you just cut in. This will allow you to constantly be working into wet paint so you won't leave lines on your walls. Use an extension pole on the roller to reach up to the ceiling if it's too high to reach with just the roller. If you have a very high stairwell, you may even need to stand on the ladder and use the extension pole. Repeat in the same manner until the walls are completely painted.
Allow the paint to dry for a couple of hours and you will be able to tell whether you need another coat. Wrap your roller in plastic wrap while you wait so it doesn't dry out. If you have any thin areas, you will want to paint a second coat. Put the second coat on the same way as the first.
Remove all of your painters tape, newspaper and/or tarps, and clean out your brushes.