First, I turned the box right side up and cut off the four flaps.
Then, I turned it upside down and cut the holes where I wanted. I used an exacto knife and a circle cutter to make my circle, but you could easily trace something like a round lid, to get a round shape.
Next, I measured down the back side of the box (the side closest to the holes that I cut)
and made marks on each edge 8" down and down the front side of the box I made marks on each edge at 4" down, then connected the dots and cut off that part of the box to get an angled shape.
Voila! A bean bag toss game!
In case you were wondering, the rules I saw on bagtoss.com said that the slit hole is worth 5 points, the round hole is worth 3 points and landing on the surface is worth 1 point.