Disassembling and reassembling a Glock model 21 .45 acp pistol will take a lot of time and effort. You need to start by making sure you are working in a well lit area. Next, make sure that the pistol isn't loaded. for more information please check out this link: http//:www.glockworld.com/category/1680-glock_videos.aspx.
The Glock 21 .45 pistol is disassembled by dry firing and pressing on the slide releases on either side. With the slide removed the barrel and spring can be removed. For more information visit: http://www.youtube.com/watch?v=IuUDBVwJ5Cc.