Summarize this article:

Position the slat support on the inside of the outer board so that the top of slat support is even with the top of the support post. Check for level and secure the slat support in place with seven 2 " screws. Position the second slat support on the inside of the sideboard so that the top is even with the top of the support post. Check for level and secure the slat support in place with seven 2 " screws. Position the end support on the inside of the headboard so that the top of the end support is even with the top of the side slat supports. Secure it in place with four 2 " screws. Position the end support on the inside of the footboard so that the top of the end support is even with the top of slat support and the support post. Secure it in place with four 2 " screws. Measure and mark the locations of the fourteen slats, creating even spacing along the slat support. Attach each of the 14 slats by drilling a pilot hole at each end into the side slat supports and securing them with two 1 " screws.

Summary:
Position and attach the first slat support. Position and attach the second slat support. Add the first end support. Add the second end support. Add in the slats.