Here's my instructions:
Step one: brace engine mounts so they don't move.
Step two: cut ass end off bike.
Step three: line everything up and make sure it is all straight and square and looks how you want it.
Step four: weld new ass end onto bike.
I may have over simplified that a little bit, but I think if you have the skills to weld a hardtail onto the bike and build a custom, then you should be able to figure it out without too much for instructions.
I think for the TC Bros hardtail, they tell you to cut the lower rails about 1 inch behind the exhaust mounting brackets, and cut the backbone (center post) under the rear engine mount at the lowest point you can.