Start by removing the ball. On most mice (mouses?) there is a cover on the bottom that can be removed by twisting it counter-clockwise. There may be other designs, but they are mostly self-explanatory.
After removing the ball, blow out any accumulated dust with compressed dust remover or just a sharp breath.
Inspect the rollers inside the mouse. There are three: two thin sticks set at right angles, and a third "idler" wheel at an angle. These will usually have a small line of crud on them. Remove this by scraping gently with an nail file, paper clip, or other sharp tool. Rotate them around to make sure that they are clean on all sides.
The mouse ball itself should look clean and feel slightly tacky. If it has a varnished or dirty appearance, it needs to be cleaned. To clean the ball, use two drops of solvent cleaner (such as Goo-Gone or Goof-Off). Then use water and dishwashing soap to remove the solvent. Rinse and dry thoroughly before reassembling the mouse.
The exterior of the mouse can be cleaned with a damp scrubbing sponge (do not let water into the mouse body). It should then be disinfected with a disinfecting wipe.
Take a moment to clean the mouse pad as well; this is where all the dust and dead skin cells are coming from.