Tearing off an old roof doesn't take a lot of special skill. Basically, you just pry up the shingles using a pitchfork or shovel. Before you begin a tear off, protect trees and shrubs near the house by leaning up plywood sheets.
Locate a construction dumpster, pickup, truck or other container close to the roof. Toss the shingles directly into the container to save from having to pick them up later and to prevent nails from getting lost in the lawn. You may even want to build a temporary chute to funnel shingles into the container. If you're using a pickup or trailer, be careful not to overload it.
Pull up any exposed nails instead of pounding them down. Otherwise, they may work back up. Replace any damaged or missing sheathing, fascia or soffits.
It's often difficult to judge how long shingling may take. Of course, the best option is to only tear off as much as can be replaced before bad weather hits. But, you may want to fasten temporary tarps at the roof peaks in case of rain or unpredictable weather.
It's always a good idea to pick up stray nails on the ground at some time during the project. A special magnetic tool for picking up loose nails can make this an easy job. However, since the roof is now exposed, time and weather conditions become important factors. So, it's usually best to do that last or as soon as the roof has underlayment.