Pour some paint on a pan with a roller frame not more than half an inch deep. On that pan, roll the roller through the bottom, making sure to distribute pain evenly through. Make sure there is just enough paint and not too much - no drips, no abnormally thick coats.
After that, apply the paint to the wall in a zig-zag manner from end to end. Return to the starting area and apply the roller again, this time in a 90-degree angle to the original application. Finish the area by rolling up and down or side to side, closing gaps, covering roller tracks, finding and fixing skipped spots and completing the coat.
Use a generous amount of paint. Don’t be stingy with the application. A good rule of thumb is: as long as it’s not dripping and drenched, it’s the proper amount.
For corners and trims, a roller may not be the best idea. A good brush is much better and will allow better application.