Using 1/4 backer board over 3/4 plywood on a small bathroom floor 5 x 5. Do I need to put thinset down before the backer board or just screw it diwn? Also using 1/2 in backer board on the tub surround do I need to use water proof fiber and a sealant on the joints? Any videos to watch?
Thanks

 

Yes to all.

Please dont take this wrong.

My advise, you probably need to spend some time, reading and even watching video's to grasp the fundamentals of tiling,

 

I would not thin set the backer board to the plywood. Just be sure the backerboard is well fastened to the plywood. Screws every 6".

 

You do need thinset under the backer board.

 

I guess I'm in error. But out of all the tiling and floors I've done, I have never "glued" or thin set the backer board to the base floor or wall substrate. And on the few that I have had to undo on a remodel, I have never seen thin set used between substrate floors. Only under the tile itself. But I have never had a problem because I did not!
I have seen elastomers or caulking adhesive used to place a floor (usually plywood) on top of a base or sub-floor.
Edit...A quick GOOGLE does in fact confirm your advice and it is recommended to add thinset.
You learn something new every day.