I love metal roofs, will never go back to shingle and have done quite a few but never steep like 8/12 etc. On the roofs Ive done, 4/12, 6/12 I used roofing jacks mid point of the whole length, a safety harness with rope and "Cougar Paws" magnetic shoes. New sheets from the factory can be quite slippery even on 4/12. If you are working from one end to the other (rake end), its best to use chalk lines to square up the first sheet and then double check every so often. What I do is just snap a line 24" from the eve end so that the line is parallel to the eve, then figure out where I want to start depending on prevailing wind (so water isn't driven up the overlap by any chance). I make a mark on the chalk line about 2" away from where the first sheet will be put down. From that mark I do either 3-4-5 or 6-8-10 etc. triangle to make the right angle to the snapped line and then snap that. From there, I snap a few more right angle lines on the roof about half way and 3/4 way just for reference so that I can measure to make sure sheets are at a right angle to the eves and so that the eve end of the sheets stay straight and not staggered. I also sometimes use a scrap 2x4 stop on the bottom to prevent the sheets from falling off the roof and you can wiggle and shake the sheet into place if its not too windy. I pre mark my screw locations on each sheet because I normally use metal over a roof deck and not purlins, when using over roof deck it keeps noise to a minimum. Most rental places will rent safety equipment and scaffolding. Take your time, those in a hurry often make dangerous situations a reality.