You should hire a contractor with demonstrated experience installing and repairing sliding glass doors. These doors are notoriously difficult to install and work with. If you hire an inexperienced contractor, you could end up with a whole host of big problems. It's best to play it safe and only hire contractors with a lot of experience.

If you live in an old home, make sure your contractor is certified to work with asbestos. There are a lot of residential properties built before the 1970s that still have asbestos in the outer walls. It is reasonably safe to have asbestos in the outer wall if it's not being disturbed. However, installing a sliding glass door may require your contractor to cut into your outer walls. If you have asbestos in those walls, then the contractor needs to be certified in how to safely deal with it.

Try to get an estimate on how much the job will cost you. The cost of the job will depend on what type of door you want, what type of house you live in, and how elaborate the job is. Your contractor should be able to provide you with a reasonably accurate estimate for you on the installation of your doors.