I asked that same question a short time ago. Check out this link. Following Andy's advice, my 14 year old daughter and I successfully replaced the door gasket and all the window gaskets. My father, who is a professional body man, was pretty impressed with the job we did!
I also want to add, the removal of the old gaskets can be somewhat time consuming. Purchase some "gasket remover" from any auto parts store. We found that using a small putty knife and a straight edge razor gasket remover work best. First, strip off all the old gasket that you can by hand. We then made slits in the gasket remaining on the window frame with the razor knife. Saturate the old gasket with the gasket remover and let it sit for a while to soften the old gasket and glue. Then carefully using the putty knife and razor knife, scrape the remaining gasket and glue from the window. Take care not to scrape or gash the aluminum frame. Clean thoroughly before installing the new gaskets.