How big is the door going to be? how thick? For what purpose is it to be used?
What materials and tools do you have to hand? Does it need to look good or just be very strong?
A door is (basically) 4 right angles welded together but you need to consider if you need a frame, if it needs to fit tightly in the hole, where the hinges will go, do they need to be flush fitting?
What will you use to lock it? And do you need to open it from both sides or just one side?
sorry, lots of questions, but the more info you give here the more you will get back
It looks like maybe 100x50 rhs mitred (cut the lengths on a 45 degree angle) and squared up with maybe 50x50 in the middle.just guessing about the rhs, so you will have to check, they probably finished it with a flap disc. 80 grit is versatile enough for that I don't know about fitting glass. trick is to measure the door and leave enough room for hinges and about 5mm all round for a good, not to tight/ loose fit. shouldn't need a plan but just the measurements. one interesting point is to check if the frame is square before you make the door