Bath tub replacement

[Fishboy]

I want to replace my ancient bath tub with a new synthetic "drop in". I've found the tub model I want, but the dealer tells me I have to build a layer of mortar for the drain and to form a base. I didn't ask at the time, and I have no idea how to do this. Does anyone know of a DIY type web site that will explain the process for installing in a synthetic tub?

Mahalo nui loa,
Brian and Mary
Lynnwood, WA\Discovery Harbour