In one sentence, describe what the following article is about:

Before you shop for a door, you’ll need to know the exact size of the opening that the door needs to fill. Use a tape measure to measure the full width of the hole. Measure from stud to stud.  Measure both the width and height of the opening. Write down these measurements so you can refer to them when shopping for a door. Before you measure, make sure to remove any boards or shims that may have been added to make the old door fit. Visit a home-supply store—such as Home Depot or Lowe’s—and look through their selection of glass doors. Make sure to bring your tape measure with you to confirm that the door you choose will fit well within the opening in your house. Glass door frames can be made of various materials, including wood, vinyl, or aluminum.  The material and quality of the door will determine its cost. A simple 6 feet (1.8 m) glass door will cost around $300. A pre-hung glass door (which is the easiest type to install) will range between $1,000 and $4,000 USD. Pre-hung sliding glass doors can be installed as a single unit. You won’t need to assemble the frame before installing it in your wall. At this point, the opening that you pulled the old door out of will likely be cluttered and rough. Remove any leftover nails or torn material. You may also need to adjust the opening. Use a carpenter’s level to make sure that the top, bottom, and side frames of the opening area all level.  If any aren’t level, you can use a shim to even out the door frame. If the frame is slightly too large for the sliding door you purchased, nail in one or more strips of 1x3 plywood to adjust the size of the opening.

Summary:
Measure the opening for your glass door. Purchase a sliding glass door. Clean up and adjust the opening.