The kind of mortar used to build mortar beds for shower floors as well as floors in all other areas is called Deck Mud, Floor Mud, Dry-Pack or maybe some other terms I don't know about. But for now, we'll refer to is as Deck Mud.

In this illustration I have used 4 parts sharp sand and 1 part portland cement. It must first be mixed dry (before water is added) until it is all completely blended together, which will turn it all to a grayish colour.

This picture shows the brown sand under the light grey portland.

There are many advantages to using the above ingredients to make Deck Mud, but for smaller jobs you can use Sand Mix which already has the sand and portland mixed together. All you need is water using the same consideration as noted above.