Noob mapper question [making a door]

i just started using doom builder so im trying to make a door. So i was following a tutorial and it said make a 256x256 room.Then make a 64x128 next to that.Then make another 256x256 next to the 2nd room.After that it says in the middle of the second room make a 16x128 room but when i try to make a line through the 2nd room i cant because the options for that line pops up. I know this is a VERY noob question but if your just gonna flame me plz dont post. Thanks very much to anyone who can help me

well, you could zoom in close enough so it doesn't highlight the line, and instead click close enough to a line intersection and click, that should snap to a spot right where you want to start the door

when you rightclick on a line, it will bring up the props.
you need to his insert to put a new line in.

Or you can place a vertex out in dead space, in then click to where you need your lines. Adjust the sides as needed. Delete the line to be sure to be that nothing is sticking out into nothing.

EDIT: Grammar

Instead of right clicking, press INSERT or CTRL+D

thanks everyone i really appreciate it