The next step (not pictured, sorry!) is to place the 37.5" 1x4 on top of this. Glue and nail into place
Now for the supports. I drilled and screwed in the 3" pieces into the 36" 2x2. I placed one on each end and then one in the middle.
I then measured on the wall where I wanted these positioned with help of my painters tape. I found where the studs were and then marked those measurements on the support. I then drilled all the way into the support with a drill bit one size smaller than my screws. This helps to not split the wood when you send the screw through.