Your biggest challenge installing a roof yourself will be the physical aspect of the job. It's hard work under the best conditions. It's unbearable work if you have to do it in blistering hot conditions or in the middle of winter.
1. The first thing to do is to decide on the shingles you'll use and obtain the written instructions from the manufacturer. The internet makes this easy to do. Just about every manufacturer has downloadable instructions for each of the shingle products they make.
2. Pay close attention to what the manufacturer has to say about installing the shingles on top of an existing roof. Talk with your building inspector and see what the code restrictions are in your town. If you are allowed to add new shingles on top of your existing roof, you will save an enormous amount of labor. Stripping off an old roof is brutal work.
3. If you have to remove your existing roof, get a great pair of gloves and wear them. Start at the top of the roof and remove any cap shingles at the peak of the roof. Use a flat spade shovel as a lever under the shingles. Special shingle removal tools are made for this purpose. They have aggressive teeth that help lift the shingles away from the roofing nails.
4. Be sure you have a contingency plan in place. Don't strip off the entire roof, leaving it exposed to the weather as a storm or a pop-up rain shower could show up at your house, and you'll have a real mess. It's possible to strip just half of one side of a roof from top to bottom and re-roof just that section. You also can purchase a large tarp to cover the house, but these have been known to blow away if not secured well.
5. Installing the shingles is not difficult. Pay attention to the instructions, and install the correct underlayment and any required starter shingles. In the old days, felt paper was the underlayment of choice. Now, there are large rolls of underlayment and specialized underlayments that prevent roof leaks caused by ice dams and windblown rain. Traditional roofing felt paper works just fine as an underlayment.
6. Use a metal drip edge at the bottom and along the sides of the roof. This product is affordable and, when installed correctly, it will really help keep everything dry. If you use traditional felt underlayment, the metal drip edge goes on the wood roof sheathing first at the bottom of the roof. You then install the underlayment on top of the metal drip edge and over to the sides of the roof. The metal drip edge on the sloped sides of the roof goes on top of the underlayment. This method helps keep windblown rain from getting under the underlayment in fierce windstorms.
7. You need to chalk lines to keep the shingles running straight both horizontally and vertically. The written instructions will show you how to create the needed offset for each successive row as you march up the roof. This offset is critical. This is what keeps water from leaking into your home where two shingles meet.
8. Pay close attention to the nailing location and the pattern in the instructions. Use the correct length nail as called for in the instructions. Remember, if there's a problem with the roof later on, the warranty depends on you doing everything as stated in the instructions. Don't take shortcuts.
9. Cutting and fitting shingles around plumbing vents, roof vents, chimneys, etc., is an art and a science. If done wrong, you will have a leak. If you currently don't have a leak around such openings, take numerous photos of how the shingles are installed under and around them as you remove the existing roof. Peel back one layer of shingles at a time from the top down and snap photos to see how the roofer before you cut and fit the shingles.