Once the drain is embedded in the mud, it will be solid. One thing the instructions are not that clear on, is that for the drain to stick to the mudbed, you need to mix more portland cement and maybe some lime into the portion placed immediately under it. Or, you can buy a bag of brick mortar (you won't use much, but it's cheap). You want the flange of that drain to actually stick to the mud. If your pipe flexes a little, you can glue it to the pipe first, then lift it up to put the mud under, then stick it down embedding it into the mud. Otherwise, you could probably stick a block of wood under there to give you something to press the flange on, keeping it at the desired height. Over a wood subfloor, you want the mud under the flange to be about 1 to 1-1/4" thick. Make sure the size of the hole is proper (I think 5"), or you won't have enough surface to provide the proper support.