I'm the designer for our haunt and it's taken awhile to finally get a good system down.
What's worked for me in the past is to, as Mysterio said, get the dimensions of the space you are going to use. That's key. From there, start with the outline, or the exterior walls, of the haunt just as a place to start. Add your entry point next (maybe even your exit just to keep that in mind).
The middle.. the actual haunt.. is the hardest part. Hopefully you're already using graph paper, but if not, get it! (I use the paper where there are 10 squares per inch making each square 1 square foot... so sometimes multiple sheets are needed). Start lightly in pencil, and just start drawing out rooms. I have my list of room ideas with me at all times and tweak and tweak til it works.
But that's just my process. I have yet to use my first draft of a layout... it's usually draft 5 or 6 that ends up being built! The first draft should look dirty... with lots of eraser marks. Make a clean version of that, and tweak that to make version 2 and so on.
Hope that helps, but that's just my process!
I'm also available for hire if you'd like. Even further consultation. Email me at firstname.lastname@example.org or message me on here!
Twisted Realms Haunted House