I have renovated literally 200+ bathrooms over the last several years.
Don't waste money on a contour gauge. Buy or borrow a lazer, home depot will price match and beat by 10% a competitor's price. Shop around.
To lay out a cut, lay your tile in place, if it's a gloss finish, mark it with a sharpie, if not, use a fine tipped pencil. Mark out the necessary cuts and go for it. It's not a tough learning curve.
Strip it to the studs, then use a level and a straight edge to make your walls plumb and true. Plumb each stud at the ends of the wall by sistering a stud on the side of it, or furring with strips of 1/8" ply. Use a 1/4 crown stapler to hold it in place. This allows 1/8" tolerances. That's plenty accurate.
Now use the straight edge to true up the studs between them, either by sistering or furring, whichever is easier.
Decide on finish fixtures, and install more backer than you think you will need for them.
Board with moisture guard if you're a cheapass, or concrete board/hardibacker if you want it to last. Use a waterproofing membrane like redgaurd on the seams.
I like premixed mortar, probably best for you too, since you'll likely over mix, or mix too much if you are mixing yourself.
Grout doesn't hide sins, unless you're slapping a LOT of grout around. Which you shouldn't be, cause it'll look like a dog's breakfast.
Find your local flooring supply house, not home depot, but the specialist that the pros go to. That's where to buy good grout, mastic, tools, caulking, etc.
You can do just fine with an angle grinder and a diamond blade if you're reasonably competent with tools, a table saw style tile saw is better, and one with a sliding tray like you can rent at hobo depot is best.
Buy a diamond hole saw kit for your small penetrations like the shower neck, and tub spout. It's worth it.
Use a tile edge product like schlueter, buy metal, plastic is cheap crap. You can CAREFULLY AND SLOWLY cut it on your chop saw to make nice mitered corners if need be.
Use tile spacers. I like the wedged shaped ones, they're easier to cheat your lines with if you need to stack em.
Check your tub before you leave the store. I'd say 10% of the home depot tubs I see have a defect or damage. Don't buy a cheap ass tub. I've had mirolins from HD that you could damn near read a newspaper through, glazing is applied by weight in the factory, which isn't a good way to get a good, uniform coat.
Good luck, feel free to PM me with questions.