You decant the paint to a tray to 1. Use a roller and 2. Stop the paint drying out and changing texture. Decant some paint to a paint kettle (plastic pot) for cutting in. It also means that if knock the tray or paint kettle over you won't lose all your paint.
You must mix the paint thoroughly before you start, unless the tin specifically states not to. Use an old wooden spatula or spoon. When you think you've mixed it enough, mix it some more. Make sure you go right to the bottom of the tin when mixing.
I never find masking very effective - not in an old house where things aren't all cleanly delineated to start with.
I know it's single use plastic, but I like roller tray liners as they make cleaning up much easier and no water needs to be used. I find rollers tend to stick to foil if you use that instead.
Get a roller extension pole for painting ceilings and high up on walls. You'll still need to get up a ladder to cut in, but it's a lot quicker and easier than going up and down a ladder, moving the ladder, etc.
Do two coats then check around the room carefully after the second coat has dried and go over any patchy bits. A small roller is good for this to make sure the paint blends in.
Get a radiator roller for doing behind radiators.