Carpeting stairs is a major project and one of the most difficult projects you can undertake in your house. That doesn't mean it can't be done. Just cutting the carpet needs to be done with a high level of care or you'll end up with unsightly edges that make your new carpeting look like it has already been walked on for years. With the right tools and some special consideration, you can make your carpet edges look like a professional did the job.
Measure the bottom stair from the back of the stair to the front edge. Measure up from the floor to the top of the stair. Add these two numbers to get the total length of each stair. Count the number of stairs. Multiply the number of stairs by the total length of each stair. This will give you the total length of carpet needed. This assumes that your stairs are the same size. If you aren't sure, measure four or five stairs to check. If you get different measurements for any of the stairs, you will need to measure every stair separately to get your total length.
Measure across the stair from one side to the other. This will be the width of your piece of carpet.
Lay your carpet out in a clean area, carpet side down. Measure the length and width of carpet needed on the back of the carpet. Chalk will mark the back of carpet with a nice visible line. Try to keep your line within a row.
Cut from the back of your carpet. Cutting from the back side will allow you to follow a guide row on the back of the carpet. Staying in a row will create an even cut.
Use a cushion back cutter. A utility knife is not recommended for cutting carpet. Utility knives can cause major fraying. A cushion back cutter is made to cut close and prevent frays.
Cut away frays. Before laying your carpet, cut away any fraying material with sharp scissors. Be careful not to tug or you will cause more fraying.