Some considerations from someone who has a few home brewed brands:
1) Heat up the metal red hot! This may sound like it will just do more damage, but consider the hotter metal will burn and destroy the skin in a shorter period of time, the result is that the outer layers of flesh heat up much faster and the branding iron spends less time in contact with the skin. Thus deeper layers of the dermis are less effected resulting in less collateral damage, pain, and a cleaner looking piece.
2) Have someone else do the actual branding for you, your body will resist pressing a hot piece of metal into your skin (trust me) so have a friend you trust not to fuck it up do all of the heating and pressing.
3) lay on your back or rest the limb being branded on the floor or a table, this reduces the chance of you squirming while being branded and fucking up the work.
4) The iron need to only be in contact with the skin for maybe .5 sec, holding it there longer won't make the piece look any better, usually to the contrary.
5) Use some commonsense and keep the dam thing clean, wipe the area with an alcohol to prep it, and dress the wound accordingly with an antibiotic ointment after the work is done. This type of work is often more vulnerable to infection than tattoos because deeper layers of skins are effected.
6) Aftercare is important if you want a brand that really stands out, irritating the wounds will ensure that the brand stands out.
Edit: It really doesn't hurt as much as you might think, bite hard on a wallet or something and don't be one of the pussies that gets high or drunk before you get work done