How to use a paint roller on walls
First, we gather our roller arm, roller pole, paint, and the roller pan/bucket and get to work. Once they have all been assembled into the desired tool for painting, the roller should be dipped carefully into the paint. When done correctly, the arm holding the roller does not get covered in paint, which helps prevent dripping and splatter while using the roller. You will also want to dip a fresh roller cover into paint repeatedly, and "condition" it in the rolling pan prior to applying the paint to the wall. Doing this will minimize the number of holidays, light paint coverage, and see through spots in your finish.
Once your tool has been dipped and soaked through correctly, roll off the excess paint onto the pan, and move it to the surface you want to cover. Gently lay off the heavy paint load that has been soaked up about half way up the wall, this will keep the paint from running down the wall before you roll it out. Next, spread the fully wet paint into an N or V shape, creating a palette of wet paint on the wall.
Now that that has been completed, start on the left side of the wet paint and roll the roller from floor to ceiling, stopping just over the edge of where your brush work stops. Repeat that motion across all of the fully wet paint until it is spread over the entire area consistently. Once your N or V shape is filled into a nice, evenly covered wall, dip the roller back into the paint and do it again, making certain to overlap the new shape with the edge of the one just completed. Generally, you'll start this procedure at the corner of a room, any corner will do. Use that point as a starting point, and work your way around the room until you get back to where you started.
The idea here will be to evenly distribute the coverage, making sure there aren't any areas with too much paint. However, that is not all there is to it. Even though rolling paint is not too physically demanding, it is easy to make a mess of. You'll want to be on guard for runny roller edges, leaving too much paint on the wall, and dry rolling. Each of these application mistakes have their own consequences, but the thing they all have in common is that using them will ruin a great paint finish.