I'm about to build a pre-slope in a shower base (floor) over existing concrete in a basement for tiles. The drain pipe is approx. 2" above. What is the proper procedure to build this up?

You'll Be Pouring A Dry Mortar Bed For The Tile. You Need To Mix The Mortar With A lot Less Water Than Normal It Just Needs To Be Moist
If Your Drain Is At 2" Id Give It A 1/2 Slope To The Drain, From All Sides.
Lets This Dry Before You Lay The Tile, Now Also Rememeber To Install Your Liner Before You Pour Your Mortar Bed.

I've also heard I can use topping cement with glue. What do you think ?