How do I layout a 45 degree angled bevel for 6 inch pipe

Are we talking plumbing or math here? If its plumbing we do not bevel pipe we use a 45 degree fitting. If its something else let me know. When welding two pipes at a 45 we cut both pipes at 22.5 degrees.

how do I layout a 45 degree angled bevel for 6 inch pipe

If I understand the question correctly you take a 6" eighth bend and, if it's drainage, give it a quarter bubble pitch on your level and install it on the existing pipe. Since you failed to tell us the nature of the job or what material you're working with that's the best we can tell you. Good luck, Tom

Install 45 degree fitting or 2 - 22 1/2 degree fittings.

Please let us know the application that you're working on so we can be of better help. Ya got us flying blind here. Tom