Be patient. Don't try to get it all done at once.
Tape or cover the glass to protect from a slipping sander.
Remove hinges and other projections.
If your sander has a hook up for dust collection, use tape to secure the hose to the dust pick up.
Tape the sander's power cord to the vacuum's hose every foot starting at the back of the sander and going back about six feet towards the vacuum.
Don't try to speed up the work by using a grit that is courser than necessary. You might take off too much wood.
If you are working on deck and need to be on your knees, I recommend a pair of double front pants with kneepad inserts. Kneepads that fit into pants are possibly the greatest invention ever. They can be washed without being removed and are always there, so you never have to worry about being uncomfortable while working on your knees.