Begin by building the two side ladders using the spacing shown in Figure A. Use a square to ensure each "rung" is square to the leg, then secure each using glue and 1-1/4-in. screws (Photo 1). Use a 1/4-in. round-over bit to soften the outer edges of each leg. Stand the two ladder sides facing each other and install the two steps and the back brace. Predrill the holes to prevent splitting.
Cut the two top boards to length and round over the top edges. Secure the two top edges to each other using 2-1/2-in. no-mortise hinges; regular hinges will also work but will leave a slightly wider gap. Position the hinged-together top boards so they overhang the sides of the legs by about 3/4 in. and the front and back by about 3/8 in. Attach one of the top boards to the top "rungs" of the ladder using 2-in. screws.