I am redoing my bathroom walk in shower stall. I have ordered a piece of marble that will butt up against the existing marble. I am adding this piece of tile to extend my shower stall by 6 inches.
I need help on what is the best way to mount this piece of marble tile.
1. Can I mount it right to the dry wall?
2. What type of adhesive do I us?

Also, in taking down my old shower stall, there are 4 screw holes in the existing marble. I need to plug those up before the contractors can come in to put in the new shower walls.
1. How do I plug up this 1/4 in holes so that no water can get into them.
2. Can you recommend a product for this job?

Use Mapei HPG first to paint drywall... or any membrane that is paintable. Then use any form of Thinset... there are rapid set thinsets available. I would just use grout to fill screw holes (matching grout) and seal with grout sealer.