"Yeah, Justin's method is what I use, too.
Always start with the mattress size and make sure you design in room to actually be able to get the sheets on the bed once it is built.
If you are using a flat surface for the mattress then this…"
"From the beds that I have built, I measure the mattress and work from there. That will give you your interior dimensions of the frame. Rockler sells non-mortise bed rail hardware that makes things easier too."