Take measurements of the location of the area you need access to before you install drywall. Take two measurements from perpendicular walls to come up with a center point.
Once drywall is installed, measure and mark a center point using the measurements you took earlier.
Compress the spring loaded tabs in the back of the access panel and measure its length and width. This will be the size of the hole you need to cut for the spring loaded tabs to fit into the drywall. Use the center mark as a starting point to cut a square pattern in the drywall using the drywall saw.
Compress the spring loaded tabs in the back of the access panel and slip them into the hole you cut. Check that the installed panel is visually square with the walls. If not, slightly increase the size of the cutout and adjust the panel.