Not hard at all.
Personally I wouldn't even start with fabric... it takes you just too much time. I personally use the egg crate grids that come from profoto. It's almost funny how expensive they are (more expensive than the softboxes) but at least they don't look improvised.
But now to what you could do to get quickly and cheap to a homemade grid.
GET CARDBOARD - I guess you already have the solution now
I've seen that at a friends studio. He just cut it into like long 2 inch stripes and then every 2 inch he would cut it in an inch. After you have a couple of those stripes you can stick them together in a grid pattern. I hope this makes sense to you so far. After you're done with that take matte black paint and give it a nice black finish.
Cheers
Boris