Damp with paint a third (1/3) or half (½) the length of the bristles. This let you to save paint and to avoid splashes.

If you are right-handed, start painting on the right side of the wall. Start to paint adjacent areas to windows, doors or frameworks.  Paint out about 5cm from the edge of these areas. 

If you are going to paint pieces of wood or plastic, apply the painting in the direction of the piece.  Paint side to side in horizontal pieces, top to bottom in vertical pieces and at slants for sidelong pieces. Wipe up any paint splash before it dries.

Clean the application tools (brush or roller) at the end of each work day. Use water and soap as cleaner for water base paints and thinner or organic solvent for solvent base paints (see the section Products).  Do not forget to use appropriate personal protection equipment.

Painting the windows: if you are going to paint frameworks, use masking tape on the glass panes. Start painting at the top and work down, painting both the vertical and horizontal frameworks. Then paint the adjacent areas to the glass panes.  Again: paint from top to bottom and wipe up drips before they dry. You will require brushes with different width, depending on the piece to paint.