Clean around the outside window frame, especially where the leak is. Any type of cleaner and sponge should work. A kitchen sponge with a hard scrubbing surface on one side may work best, but that may be too abrasive for your taste. Make sure the surface is as clean as you can get it.
Remove old caulking from the window. You don't have to remove all of the old caulking material, just some around the edges near the leak, especially if it is already starting to wear off. Peel off the material slowly with a razor blade, making sure you don't cut yourself, scratch the paint or cut into the fiberglass siding.
Cover the window frame and the surrounding siding where you don't want any caulking material. Masking or mailing tape should work. You should border the tape about 1/8 of an inch away from the edge of the window frame.
Insert a tube of caulking material into a caulking gun. Clear, flexible silicone sealant works best. If the tube is brand new, you must cut a hole in the tip of the tube and then puncture the inner seal by poking something like a screwdriver down the hole. Do this before you place the tube in the gun.
Apply the caulking material along the window edge with the caulking gun. You could do this just around the leak, but running a fresh caulking seal around the whole window frame may prevent future leaks. After applying the caulk, you might try pushing it further into the cracks with your fingers. Wearing rubber gloves will help.
Let the caulk dry for 24 hours, then remove the tape.
