The shower curb is constructed of three 2×4’s which is then covered with the shower liner, and then covered with a metal lathe.  On top of that I’ve put about 3/4″ of mortar.  Forming the curb was actually easier than I expected.  Using a pair of wood strips  on the top to keep it level and make a nice sharp corner, I was able to get a reasonably decent looking curb.  I found that I had added a bit to much water to the mortar, which meant that the sides of the curb kept slouching down.  However after waiting a bit for the mortar to firm up and dry out, I was able to get it to stay in position and got a nice flat top, with nice sharp corners.  I’ve also put a bit of a slope on the top so that any water that gets on it, flows into the shower, instead of out of it.  I did notice after the curb had dried that the right side was a bit thicker than the left, but its barely noticeable and shouldn’t cause any problems.