Decide where you want to put your bar and how you want if orientated in the room. Measure off of the wall how far you want the bar to come out. For me, I came 7 feet off of the wall and decided that was good enough. You will need to cut two pieces to length (USE MITER SAW FOR THIS CUT, GET HELP IF YOU NEED ASSISTANCE CUTTING). So for my example I would cut two (2) 2X4's at 7 feet. This will give me what is called the bottom and the top plates.
In the picture the bottom and top plates are what you are cutting now. Don't worry about the picture being upright at this point. We will deal with that in the next step.