@spotiche5- The first thing that you need to do is to turn your chairs upside down to determine how the upholstery is attached. Whether it is stapled down, attached with screws, or tacked into place, you will need to attach the new material in the same way.
Next you will need to purchase fabric that is either waterproof or labeled for indoor and outdoor use. Measure the chair surfaces that need to be covered first so you get the right amount of material to do the job.
You will also need to get new tacks, screws, staples or other hardware to secure the new material into place. You will also need the tools required to attach the type of hardware you will be working with.
Finally, set aside at least a few hours to re-upholster your patio chairs. If you follow the pattern of the old upholstery, you shouldn't have any problem replacing it and attaching the new material.