I build on grass becuase I had a dinning table that was made out of glass.lol
It was the flatest surface i came across.
I had a work table which I though was flat....not.lol

Later I used a piece from an old stereo consel.
I simply tape the plans to the underside of the glass...No more wax paper or messing around with plans to makesure the plans are flat.,flat.lol
I simply just tac the end of the spars to the glass. Sometimes I might need to tac a little piece of wood for the center of the spar to keep it striaght. It acts like a pin. (butt it)
I simply tac those little tabs of the ribs to the glass.

I do the samething for some fuselage...simply tac the formers. Most model starts off with squaring the front and rear former of the wing.
I leave enough over hang...for the firewall's top. I tac the tail of the fuse to makesure it stays striaght.
Most GP model are build upside down with a flat top deck...then you add the fuslege's upper deck later.
Some models you don't even build the fuselage on the plans becuase of the inter lock parts. Such as the eagle II or 4*.
You simply use rubber bands to lock eveything into place.

I still makesure the sides are stright up and down by butting a square or triangle to check.

I use wieght as i would, if i was building on any other surface.

Easy clean up with a razor blade.

I also use glass for cutting my covering film. Easy on the blades. The blades easily glide on the glass without getting dull if i was to cut on wood.
If I want to do a flame job, i simply make a stencil out of newpaer or whatever.
Tape the film to the glass..then tape the stencil on top..then trace/cut....easy.

A steel ruler or yard stick (leveler type). This helps keeps everything nice and stright. The leverler won't bow.
Steel rulers or 90...so the blades can't cut into it.lol