Lay 2x4 pressure treated flat on existing concrete fasten with ramset or concrete screws that is inch and half to inch five eighths then add 5/8 to 3/4 plywood to get desired height the concrete screws are called tapcons, there are other cement fasteners just whatever works best.

You can use a thing called backer rod make just for prefilling large gaps prior to caulking they come in different thicknesses and are shoved into gap prior to caulking, gives caulking something to hold onto and if this is a wet area 100% silicone needs to be used. When applying silicone put blue painters tape at the edges of where you want silicone to stop,install silicone then Cann lick finger and wipe one side to other to make sure it's in good. Let set an hour and peel blue tape off should be nice neat line of caulk.Wait at least 12 hours then use shower without fear of water damage.