Stellar weld on sub-frames use the rear master cylinder mount as a guide. They're really simple to do. Once you bolt it at the master cylinder it almost lines itself up. You bolt the tabs to the sub-frame, center it up and tack weld it. Then remove the sub-frame and weld the tabs up.
EDIT: I like bolt on sub-frames because in the event you pile up the bike you can just buy a new sub-frame and bolt it on. You don't have to hassle with grinding on the frame especially after powdercoating.