How do I attach natural stone to backerboard in a shower
Normally the stone is applied directly to the backer board once the pieces are set a grout is used between the stones (today I would recommend epoxy grout). After grout a sealer can be used , I have done a couple jobs like this and do not recommend for a shower area. Even if well sealed natural stone picks up the soap scum and can be a pita to clean. I did a job for a guy that cut his stones , I thought it looked good and was remodeling a bath at my own home so I used my tile saw and did that shower it looked great, about a month later my wife was complaining about it, by 2 months I had to remove it and tile it. Several months later the customer called and asked how much to demo and replace he had the same issue but he had some fungus growing. 

I have done other stone work and it was fine but in the shower it was hard to clean.