First of all no matter how much you like the program "Sweat Equity" keep in mind that Amy Mathews is an idiot. She has no talent that wasn't devised by a writer somewhere and a lot of her ideas are nothing short of just plain stupid. She's a celebrity not a DIY'er.
Waterproof the wallboard. Redgard can be ordered from Home Depot if it isn't on the shelf but I'll bet it's on the shelf. Go look in the wood floor adhesives isle and I'll bet you find it. Home Depot doesn't know what it is so they don't know where to display it. My local store is probably the only Home Depot in the world that displays Redgard in the tile isle, thanks to me.
They aren't any smarter than Amy.
Use something for a straightedge and place it on two stacks of tile.
Elevate the tiles to where they need to be then level the straightedge and begin your second row of tile stacking it on the straightedge. Put the first row in tomorrow after the second and all subsequent rows have dried.
In place of Custom's Redgard you may have access to Mapei's HPG, Laticrete's Product # 9235, Laticrete's Floor and Wall Waterproofing. All basically the same thing. I know you can find one of them right there where you are.