Begin by positioning the header and footer on the outer stringers and then place the middle stringer in between.
Be sure to drive three 16d nails in each. You'll find it easier to do that with the parts upside-down, but take care not to break any pieces, or you'll have to cut new ones.
Flip the entire project over and lay out the treads on the stringers.
Recall that there’s an inch overhang on both sides of the stringers. Here’s what you can do: nail in one of the sides first, with the correct overhang, then move onto the other side and try getting it as close as you can.
The board bender can be very helpful here but don’t push it too much, or you'll break the stringers. After nailing the outside stringers, the middle stringer is pretty easy to fasten.
Don’t forget; 3 nails go into each stringer. Now is the time to add the legs. You want to have another person hold the legs in place as you nail them. Alternatively, you can use scrap blocks.
If you want the legs to offer your free-standing wooden blocks the right amount of support, you’ve got to ensure they’re correctly attached. Put around 4 on the side of the leg that touches the header and the stringer and about 2 through the top of the tread.
As you position your legs, it’d be better to have the let-ins face inside than outside, for the sake of beauty. And when nailing the let-ins, nail 1 side, and then fasten the other side from the opposite direction. You’re driving in 2 nails on each side.