Good Question that needs a very precise answer with some good resources.
Things You'll Need : Water, Rag, Canned air, Cotton swabs, Glass cleaner, Alcohol, Tissue.
Move the telescope indoors, if applicable, and place it on top of a flat work surface. If the work surface is dusty, clean if off with a damp rag prior to placing the telescope on it.
Lift up the end of the telescope so that you can see the main lens at the end. Press down on the nozzle of a can of canned air and spray the lens thoroughly to remove all dust and loose debris. Concentrate the air in the circular edges of the lens since this is where most of the dust will accumulate.
Set the telescope down and lift up the other end where the eye piece is located. Spray the eye piece with the canned air, using the same method as before.
Examine each lens to locate any smudges or stuck on dirt. Apply 1/4 tsp. of glass cleaner to a cotton swab and blot the surface of the debris gently until it comes loose. Do not rub the cotton swab across the lens since the debris may scratch it.
Dip another cotton swab into alcohol and place it at the center of the main lens. Begin moving the cotton swab in a circular motion around the lens very slowly. Switch cotton swabs three to four times during this process, so that you do not simply move the dirt around the lens. Repeat the process to clean the lens on the eye piece.
Fold a tissue into a sharp point and apply 1/4 tsp. of alcohol to the sharp point. Place the point at the edge of the main lens so that the point is positioned at the edge of the lens and the plastic. Move the tissue around the perimeter of the lens to clean out any debris that is stuck to the edges. Repeat the process to clean the eye piece in the same manner.