It is important to clean and paint the ceiling completely before you work on the walls. For both ceilings and walls follow the same steps given below.
Sand the wall by sweeping sanding paper from one side to another and from top to bottom. Apply medium pressure to make sure that you are not washing out or scratching the wall off.
Use a scraper to remove bumps and drips. Then roughen up the surface by using a dampened coarse-grit sanding sponge. Keep a bucket of warm water nearby, and continually rinse the sponge. Finish with a dampened fine-grit sponge. Use a wet/dry vacuum to clear all the dust from the walls and trim. Wash the walls with a sponge, using warm water and dish soap. Scrub greasy or waxy spots. Wipe everything down one last time with clean water.

Seal any gaps where molding meets wall. Wet your finger and smooth the sealing with even pressure to push it into the crack and leave a crisp edge. Using a putty knife, fill any small holes in the walls. Use patching compound for plaster and joint compound for drywall. Sand the filler smooth with 120-grit sandpaper. Prime the patched spots (or the whole wall if necessary). Sand primed areas with 120-grit sandpaper and wipe clean of dust with a damp sponge.

Priming your walls before painting is an important part of any painting project. Primers are specially designed to adhere to different types of surface and receive your top coat of paint. Priming helps seal the wall and can help prevent mold. Start by painting in 3X3 foot sections. Roll in one section at a time, moving from top to bottom and from one side of the wall to another. With a fully loaded rollers, work top to bottom, rolling back and forth across the wall in a series of V or W shaped strokes until the section is covered.

After the primer is completely dry, lightly sand away bumps, ridges and other imperfections using fine-grit sandpaper. Wipe the wall clean with a damp towel or sponger and allow it to dry.

Dip a 2½-inch angled brush into a bucket of paint, loading the paint only a third of the way up the bristles. Tap off—don't wipe—the excess on the side of the bucket. Use the brush to cut in a 2 to 3-inch band of paint at all corners, against the ceiling, and next to molding; this will give the roller some breathing room so it doesn't bump against adjacent areas.

To cut in, run a line of paint along the wall about an inch away from the edge. Then turn the brush onto the bristle tips, and press down slightly so the longest bristles gather into a point. Use this point to draw a careful line of paint right up to the edge where wall meets trim. Once you have a clean line in place, level out any heavy areas or drips, then move on. In order to keep a wet edge, don't work in too large an area at one time.

Once you've cut in around an entire wall area, use a roller to fill in the field. Dampen the roller before using it (with water for latex paint or paint thinner for oils). Dip the roller in a tray filled with just enough paint to reach the grate. Roll it back against the grate to distribute the paint and squeeze out the excess. Make sure the roller is covered completely before painting with it.

Roll a W or M shape on the wall to distribute the bulk of the paint. Then use overlapping vertical strokes to spread paint evenly between the lines. Continue painting the wall in this manner until it is covered. Overlap a bit of the cut-in edges to blend away any visible brush marks.

With broad moldings, such as baseboards and wainscoting, use a wide, straight-edged brush to paint the bulk of the trim.

Then, using a small, angled sash brush (1- to 2-inch), finish by carefully painting a straight line along the edge. Hold the brush on edge as you did in Step 5, and let a hairline of paint carry over onto the wall to make up for any imperfections on the molding itself.