I am a big believer in applying a liquid paintable waterproofing membrane before tiling inside of a shower tub enclosure.
This prevents water from being absorbed into cement wallboards and the mortar bed. It is extra insurance against a leak.
I prefer Redgard but they all will work fine. I always use an Oatey PVC pan liner in addition to liquid waterproofing.
I always apply 3 good coats. It is good to use the reinforced fabric at all corners also although I have never done it. I always mesh tape and thinset all seams and corners.
Thanks!
It is really important to clean all surfaces really well before applying.