To get my sides to be the right height, I used blocks of wood under the front legs of the sides. I will point it out to you in a min.
Attach the sides to the headboard using "L" brackets.
Then you will need a 1X2 that is the length of your headboard. Screw it directly into the headboard. This will be where the boards go to hold your mattress. (it is the lighter wood that is horizontal on the headboard.)
Now, sand it like crazy.
You will need some 1X6s for the next part. Measure the distance between the two legs and cut your 1X6 to that size. It is going to be the front of the bed. I used "L" brackets to attach it as well. Then, just like you did with the headboard, attach a 1X2 directly into the 1X6. It will be the front part of where the mattress boards will rest. Measure the distance from the front 1X2 to the back 1X2 and that will tell you how long to make the boards to support your mattress. I used 1X6s and 6 seemed to be just the right number to give the needed support.