Think in terms of a clean room. Dust and dirt are the enemy.
Make sure you have enough room to move around while you paint. Put up plastic walls, ceiling, and floor. If the wind is blowing hard, don't paint. Give your self a work surface inside the plastic. Every time you go in or out dirt is coming in. Your clothes shed lint. It's amazing how much stuff floats around waiting for wet paint.
The paint covers the surface you give it. Good, bad, or ugly, if the surface looks that way before paint it will look that way after. No matter how much time you put in to preparation, when you are done, really done, spend another hour. Take a bright, hand-held light and put it parallel to the surface and look for bumps. Not only the small variety, but long hills and valleys too.
Your hand is the best device for determining surface finish.