Measure your bed. Mine is full size and measures about 52" wide, so I wanted the headboard to be slightly less wide so it wouldn't show on the outer sides of the bed. I settled on a 48" wide headboard. Both of the outermost posts are 2" wide, and the inner bars are each 1" wide. I wanted 5 inner bars, so I took 48", subtracted 4" (the width of both outer posts) and then subtracted 5" (the width of all 5 inner bars combined). That gave me a total of 39", which I divided by 6 (total number of gaps between each of the bars), giving me a 6.5" gap between each bar.
I found it easiest to work from the inside out. To do this, first cut out all 5 inner bars. All of my bars are 1" wide and about 32" tall. This is a matter of personal preference depending on how tall you want your headboard to be. These pieces do not have to extend all the way to the floor, just behind your mattress. (The curved parts that connect the bars are separate pieces you will cut out later.)
Find the center of your bed on the wall and using your level to keep things straight, stick your first bar to the wall. Measure 6.5" out on either side of this bar to attach the next two bars and so on until all 5 bars are stuck to the wall, making sure to keep the tops of all bars level.
Next, cut out the curved sections that connect the top of the inner bars. Make a U-shaped template that is 8" wide and as tall as you want your arches to be. The arches will overlap each other and the tops of your inner bars. Once you have all 6 cut out, stick them on the wall, carefully lining them up with the tops of your inner bars.
All that is left are the decorative accents. The ones on the inner bars are 2" wide by 1" tall (bottom) and 2.75" wide by 1.5" tall (top). The ones on the outer bars are 3.75" wide by 1" tall (bottom) and 5.75" wide by 3" tall (top). To attach, measure down from the top bar 12.5" (or whatever looks good to you) and attach each bottom accent to each vertical bar. The top accents get stuck on directly on top of these, using a level to keep them neat.