There're so many videos on youtube showing how to do it.. and much more on how not to do it..
I shall be frank here.. The only thing you need is safety-precaution, guts and confidence..
1) Learning 'what'(What are the procedures involved?)
2) Preparation of learning 'how'[Sraplock, Duct tape, Cheapo-guitars, strong strap(nylon), shirt with collar(fold up during spin), Army t-shirt(with shoulder pad), gloves(catching the spining guitar)]
3) Make up your mind (You should tell yourself something like.. "It's easy, im just gonna throw and it's definitely coming back--No problem")
[**Note: There are chances that thing might go wrong... but you need the right mindset to do it first]
4) Learning 'how' (With all the 'what' you learn from 1st step, perform guitar-spin)
There are two possible outcomes.. as you know.. with all the protection gear.. an undesirable outcome might not be severe enough to kill you..
5) Two quotes one should learn "Practice makes perfect" and "Noone is perfect"
Summarising both.. mean you should practice and bear in mind there's a chance that you'll screw up.
Yepss that's it.. I think in arcade there's this "GuitarFreak", maybe one can drill there.. =D it's kinda 'not-yours' anyway LoL..
PS. RyanZ why not spin with LesPaul? The weight is nice for multiple spins XD~ and also the weight is enough to screw itself up in case it flies =D