Title:           pony petplay IRL
Author:          Anonymous
Pastebin link:   http://pastebin.com/2Pbwqr8t
First Edit:      Monday 12th of May 2014 02:22:55 AM CDT
Last Edit:       Monday 12th of May 2014 02:22:55 AM CDT

Voice training:
http://pastebin.com/dgipdsge
 
Pony self Hypnosis:
http://myswordisunbelievablydull.wordpress.com/2013/02/21/pony-hypnosis-project-pony-gonzo-intro/
 
Resource for ponyplay equipment:
http://www.cpony.com/home/
 
The rest is a combination of prosthetics and costuming.  The voice training makes you sound like a mare, the hypnosis makes you feel like a mare, and the resources and costuming helps you look like one.  I'd use the pony hypnosis general pastebin, but I don't know where that is.