Ok, so I'm looking at getting a guitar strap for the guitar I don't own. Anyway...I want to get one off eBay of something. We don't really have any music stores around here. Can I put it on myself? Is it hard? Do I need anything other than the strap? I don't know anything about them, as you can tell.

Oh, and is there any specific length I should get? Or are they all kinda the same?