This is a display shelf I saw in West Elm that went for $100. My version costs about $10-20 to build, depending on wood and stain prices.
Step 1: Drawing it out
Using those measurements I drew out the shelf outline on a 1/2" piece of plywood, about 2'x4'. (3/4" would work too, to make it more sturdy). You can get this at any Home Depot or Lowes.