Schluter makes a conversion drain...you take the top of the clamping drain off, then add KerdiFix and bolt the conversion piece to the existing drain, then you can install the Kerdi Membrane. You need to cover the wooden curb with some CBU before you can put Kerdi over it - it is not designed nor will it bond properly long-term to wood. The preslope must come up properly to mate with their conversion drain's flange. You really should use the Kerdi to at least the height of the showerhead. Because CBU will absorb LOTS of moisture from the thinset, use a wet sponge to both clean the dust off it, and to add some moisture so it does not suck the moisture out of the thinset. Otherwise, it is very hard to get the membrane to stick properly. The thinset mixing instructions often have a small range of water to add...when installing Kerdi, use the maximum they allow. It should be closer to mayonnaise verses peanut butter consistency, but should still hold a notch and not run. If you get at least a 2" overlap of seams, and bed it properly in the thinset, it is waterproof. I'm biased...I like having a membrane surface waterproofing system verses a liquid. For a first timer, I think it's too hard to ensure you get the proper thickness without pinholes when applying a liquid. Too thin, it doesn't work...too thick, it has problems, too.