Rollers enable the average unskilled person to produce an excellent coating much faster and easier than by brush application. By following these suggestions regarding fundamentals to rolling, the finished job should turn out to everyone's satisfaction.
Use a roller with a short nap cover; 1/4" - 1/2" for smooth surfaces and a longer 3/4" - 1" for rough surfaces.
Select a quality cover, lambskin is best. Don't make the same mistake most homeowners make, they buy the best can of paint available and then buy a cheap $2 cover to put it on with. You would be better off doing the opposite!
The same applies to paint brushes, there is no such thing as a "good" $5.00 paint brush. I am not affiliated with Purdy paint brushes but in my opinion you will not find a better brush on the market today. They provided me with 30 years of good trouble free service. Expect to pay $15 - $20 for a decent polyester 3" brush that if cleaned well will last a lifetime.
Roll off excess paint on the roller grid to get rid of excess paint and also distribute the paint evenly.
Lift the roller at the end of each stroke to produce a uniform coating.
Always roll from an unpainted area into a painted one, finish off in the direction into the window from the entry point into the room.
Apply less pressure on the roller when it is first dipped in the paint and more pressure as the paint is being transferred to the surface. Never run the roller until it is completly dry as it is difficult to get it loaded again. I suggest you determine how far you can get with one loaded roller full and try to cover the same amount of area with each dip.
Don't spin the roller at the end of a stroke or the paint will be sprayed in all directions.
Roll in a straight line. If you twist the roller, it will produce lines "roller tracks" which must be re-rolled.
Don't roll too fast. This will result in air entrapment which will produce bubbling of the paint and flip the paint all over.
If the paint is bubbling, roll slower - or add thinner - or use a shorter nap roller cover.
Don't try to spread the paint too thin, or after it dries another coat may be required.
It's always best to roll out of an empty 5 gallon can equipped with a can grid. Fill can about 1/3 full. Pans are inconvenient, tip over very easily and I am convinced they were invented by the owner of a major carpet company because they WILL be stepped in or fall off the ladder if you try to use one to cut in with.
If the paint runs, it is being applied at a heavier viscosity than normal. Roll off more of the paint on the can or pan grid. If the paint is too thin due to addition of thinner, more non-thinned paint should be added in order to thicken it to the proper application viscosity.
You should always start out by painting a 2 or 3 inch strip at the top where the ceiling meets the wall starting in the corner. This strip is called a "cut in" and it allows you to roll close to the ceiling or wall. a 4' or so threaded wood pole screwed into the roller frame saves a lot of up and down the ladder.