I followed a plan for building the headboard and footboard. I did change it slightly to accommodate my mattress and make the footboard taller.
For the frame portion, I attached 2x4s to the side boards using 3 inch screws. Then I cut 6 slats (probably overkill) and attached them to the 2x4s using the same 3 inch screws.
I'll be honest - I had no clue what I was doing with the frame portion, I was just winging it. But it turned out incredibly sturdy!
P.S. The finish is Varathane Dark Walnut & Minwax Clear Satin