There are a few ways to install a metal roof. Although I recommend this method.
On a conventional shingle roof, first, tear off all shingles, and remove all nails sticking up. This can be done with a roofing spade. You will want a smooth flat surface to work with. The steeper the roof pitch, the more difficult it is to walk on.
I am assuming you have basic carpentry knowledge. After you have swept all debris of the roof, the first thing you will need to do is install a # 30 felt paper to cover the entire roof. Start at the lower corner of the roof and run horizontal leaving the bottom edge hanging off the edge of the roof about inch inch. I use a staple gun or slap stapler and 3/8 to inch inch staples to attach paper. Staple the paper about every 12 to 16 inches side to when run is completed, start a new run just above the other allowing about a 2 inch overlap. Most of the time there are lines on the paper for you to go by, just overlap to the first line. Continue this process until you reach the peak of the roof. Overlap any valleys or ridges about 2 to 3 ft. This does not have to be pretty as it will not show. But cover all roof areas so moisture can not penetrate.
Now for the important part, getting the 1st metal sheet started straight and I use this procedure. Each piece of metal roofing has a lap edge and a lapped edge. The Lipped edge has a bout a inch inch flat spot before the raised ridge, and the lap edge does not. You should always start with the lapped edge away from the direction you are going so that when you install the second piece, the lap edge will fit snugly over the lapped edge for support. First, position the first piece of metal about 1 to two inches from the peek of roof, depending on how you measured it, I like to leave about of of an inch hanging over the bottom so water will flow into the gutters. You will want both sides to measure the same and flush with the outside edge of the nose board going up the roof edge.
Now go down to the other end of the roof and measure down the same distance from the peek and mark a line. Now chalk a line from top of panel to the line you just marked. This will be your line in which to align the top of the panels. Now slide your next panel into place, Top to the chalked line and the bottom should match up to previous panel edges edge. I usually install a couple screws at the top most part of the panel just beyond the ridges in the metal. Then apply 2 or 3 screw up about 6 inches from the bottom of the panel to hold in place. (These should be measured to keep uniform.) Continue this method until you have reached the end of the roof. the final piece to fit. For this you can use a rock blade in a circular saw, or a pair of tin snips, but if you choose this method be sure to wear thick leather gloves. getting a few nicks .If it is necessary to cut around vent pipes or vents, these cuts need to be clear and tight as possible.
When all of one side of the roof is finished, I usually go back and screw it down, although you may choose to do this as you go if it is extremely windy. I place screws 2 ft apart vertically and to the Right of the ridges if your going left to right, (ridges are approx 9 inches apart on conventional metal roofs, place screws about inch inch out of ridge), or opposite the lapping ridge. You can use a chalk line to make sure you keep your screw rows straight. This is only necessary to do on horizontally. When this side is screwed down completely, repeat on opposite side of roof.
Now you are ready to install closures and ridge cap. A closure is a piece of neoprene foam used to keep moisture and bugs out of the ridge cap. You will want to measure the width of you ridge cap and split the difference on each side of the ridge and pop lines (Chaulk lines) on either side. Other words if you have a 10 inch wide ridge cap, pop a line 5 inches down from peek on both sides of peek.
If you have done everything right you should have 2 to 4 inches of metal above this popped line.
Now take out your closure strips and place about inch inch above the line all the way down the roof on both sides. They will interlock on the ends. Cut of excess at the finishing end. Now place your ridge cap over the closures and being sure to stay on the popped lines for trueness. Install a screw down through the cap into a ridge on the metal roofing (not in the flat), you'll be installing a screw about every 9 inches apart into every ridge on both sides, cut last ridge to fit.
Coming soon, part 2 will be cutting valleys and hips, and installing trim accessories. Good luck and have fun, but most of all be careful, and be sure to take every safety precaution.