It is very easy. There are four screws and maybe two or three plastic clips for the door panel. Check miata.net for the exact writeup on removing the interior door panel. After that pull back the top corner of the plastic sheet that protects the door. Watch out the for the sticky stuff that holds in on, it can get everywhere if you aren't careful. Wear some latex gloves and clean out the rails for the windows with your fingers. There is bound to be some dried grease in there. Then use some new grease and spread it on the inside of the rails. Roll the window up a few times to spread the grease around and see if you need to add more to wipe some up if you put too much. It maybe took me 15 minutes for each side. Others will be able to recommend the best grease as I can't remember the best right now.