It's totally self-explanatory, and a lid from a paper box would actually be perfect for this, but I took pictures nonetheless. :)
We cut up a cardboard box and actually used one of the side walls as the bottom. Since we are building Legos on this surface, it's better for it to be as flat as possible.
Using duct tape, we taped up the sides.
I taped over the cut edge and down to the bottom, to make the outside a uniform surface as well.
We painted the inside of the box white.
And we painted the outside of the box blue. (And I thought, If I'd used painter's tape, we wouldn't be doing this... but we enjoyed the extra chance to paint.)
Then I drew some roads in the box, and Ephraim added some grass, trees, and a stop light. He was concerned that none of the streets had traffic signals.
Then, he got to work building Legos in his new almost-sister-proof world!