For this design you don't need particularly straight lumber, but it helps. You'll want to take one of the 2x4s and cut it in half. The other two 2x4s you'll cut into 3' sections, with 2' of scrap left over.
Nail the two halves together to form a T, as shown.
Nail the 3' legs to the T you've constructed, as shown. Make sure that the legs are below the level of the top 2x4. You want your work to lay flat all the way across the sawhorse -- if the legs stick up too much it won't.
Now nail the two 2' scrap pieces to stabilize the legs, as shown. And finally put a 2x4 across the bottom to stabilize the legs. You can use the sawhorse without this last step -- and it makes it a 3 board design instead of a 4, but you'll find that it's a lot more durable with the bottom brace.