If your old patio slider needs replacement, the most important thing is to measure correctly so that your new door will fit properly. Patio sliders generally come in standard sizes, so it should not be a problem finding a new door that will fit. Custom-size doors can be ordered too, based on your careful measurements.
Insert a utility knife around the door frame and break the silicone seal. Cut away all the old silicone and paint attached to the door trim. There may be several layers, so this part of the job may take some time.
Remove the slider door trim with a crowbar. Be careful not to damage it. Pull out any nails with the hammer. If the trim is held on by a screw, use the cordless drill and screw bits to remove it. Save the trim piece so you can put it back later or use it as a template when you cut down the new trim.
Measure from stud to stud, using a tape measure. Stud to stud refers to the 2-by-4 framing all around your slider door. Your door actually attaches to this framing. Measuring stud to stud is known as the "Rough Opening" or RO measurement. You'll need to know the RO measurement when you purchase or order your new door.