The only way to stop a rear main oil seal leak is to replace the seal. A Mitchell, Chilton, Haynes, dealer auto repair manual or a specialty repair book for your make of car will have step-by-step instructions so you can judge for yourself the difficulty based on your level of experience. Also, most public libraries will have books on the subject you are searching for or you can go to http://www.2carpros.com/store/index.htm
for specialty books the might also assist you. Just click the hyperlink above, scroll to the bottom of the page and then click on your make of vehicle.