Let's see... First, look at this site... this will help you to see what is involved in doing this job correctly. How to Build a Shower Pan
Then, yes, you need to install a liner (as presented at the site provided) EVEN on a concrete floor *sorry*
You will need to pitch the liner (see website) using concrete and tar paper, Then install the liner, then install drain into/with the liner pan. Then you will TEST the liner
by plugging the drain with a test ball or similar (Cherne makes this test ball... sold at plumbing supply house... see pic.#1)... fill pan to threshold level or near, then mark that height with permanent marker and 24 hours later should still be real near that line (some evaporation is expected, but mark should be damn close to water line).
THEN you can pour the concrete... will need to be concerned about weep holes around drain (will explain more as you go along) and adjust the drain height at that point for your tile (see pic.#2... this is type os shower strainer used with vinyl/PVC liners.).
YOU WILL NEED TO HAVE THE TILE YOU WILL USE FOR THE SHOWER to determine finished height of strainer.. I recommend a tile about 2"x2".. does great job accommodating the slope of shower floor.
Now, slope... I like 1/8" to the foot....some go 1/4"... some go 1/2"...I hate 1/2"/ft. pitch in shower... feels like I am going to fall into the drain... stick with the 1/8"/ft. (my opinion).
Also, note that a shower drain is required to be 2" pipe. You have a tub that was probably roughed in 1.5 inch pipe. YOU SHOULD re-pipe to 2" pipe all the way back to the three inch pipe for toilet... but if not, you will need to increase the pipe size in order to install the right shower strainer.
There... *whew*... This should get you started. Let me know what you are thinking and if we can help anymore. If this helped please click on RATE THIS ANSWER. Thank you.