I remember wondering this when I built my first tile shower. After questioning numerous people and getting numerous suspect answers, I finally found an old-school tile man who told me to use a sand-mix.
Sounds straight forward enough, but after looking at every bagged cement product in the home improvement store, I concluded, erroneously, they didn’t sell a sand mix. The manufacturer carried by that store refers to their sand-mix as Sand Topping Mix. A slight difference in the name made all the difference for a novice who didn’t know what the heck he was looking for.
If there’s no bagged sand-mix available in you area, you can make your own. Dry mix 1 part Portland Cement with 4 parts sand.
When it comes time to install the mud bed in your shower pan, add water sparingly. Unlike concrete that gets poured, a mud bed gets packed. The mix has to be dry enough to hold it’s shape. When the mix is the correct consistency, you should be able to pick up a handful and compact it into a ball, think snowball, that holds its shape but isn’t sticky, or gooey on the outside.