Ok, you have your old pan out and removed. Did you also remove all the mortar or concrete?
You should remove the tile up and all around the shower enclosure about 3 feet.
This is what I did to my shower enclosure when the pan began to leak. I live in a home with a slab foundation and the pan was leaking and the water was seeping into another room and rotting my floor mouldings and rock.
Again, I tore out the tile floor and removed the old concrete or mortar flooring. I fixed my drain.
I removed about 3 feet up of tile and checked and replaced the green board area. I used green board not cementious board.
Now, I bought the thickest and heaviest plastic pan liner membrane and laid it on the floor with the upper reaches going up about 3 feet up each wall area. I made sure that the hole for the membrane going to the drain was sealed and allowing for water to drain down the membrane to the drain area and leave the home.
I stapled the membrane to the wall studs and made sure it was smooth and without any seams that would make the rock stand out.
Then I set up the green board on the wall areas, I recommend that you use cementious board nowdays.
I taped and bedded the areas with water proof tape and mudd.
With this in place, I then mixed and troweled my mortar base. This has to be done with exactness. You want the mortar floor to sloop or fall from the sides to the drain. Not too much of a fall or it will be hard to stand, but just enough to allow nice and smooth drainage of water to the center drain.
Let it dry thoroughly and take more time for drying than needed. This has to be properly cured out.
Now, when this is done, its time to re tile the area and grout and silicon seal the grout areas. Then when dry silicon the corners.
Remember the drying times for each stage and adhere to them. Mastic takes 24 hours, before grouting, grout takes 3 days before sealing. Water cannot be used in the stall for a good week after laying tile.
You should be set.
you got into a big job. Good Luck.
slab foundation shower pan jobs are different than pier and beam pan jobs.