Many ways, you can use wood, metal, those tube things that connect together, some people are probably going to post blueprints. I just wing it though...
its pretty simple to do infact if you know someone who is skilled at weilding and you can find an old metal bed frame thier perfect for makeing a cage. or you could do what i did and just make one out of wood and wire mesh.
I make another picture frame shape, use hinges, and then an eye screw, with a hook that holds it closed. Bars on top and bottom of that, unless the door is an entire side, then connect mesh.
Do you have wire cutters all you need to do is cutt out a square for the hole cutt a bigger square from your left over mesh and place it over the hole and tie one side of it to the cage with wire.Use a bungee cord to keep closed. i have a photo if you would like to see.
i have a photo of the entire cage sorry not just the door im away but will be back soon if you still need to see but its not that hard. First you should assemble what ever cage you are then make doors. Just cut out the size door with pliers and cut a bigger section of mesh from your roll and use some wire to attach it too the cage.
Its basically you make a hole then cover it up with a bigger section of mesh