The basics for a beginner? Methods vary, but a very basic overview for using 6-mil poly as the barrier:

If you want to use poly behind cement board, then that poly can be your water protection. You don't need any other waterproofing (RedGard as you mentioned) in the assembly.

Have the tub properly installed, the tub flange should be fairly tight against the wall studs.

Staple 6-mil poly to the studs. Allow the bottom edge of the poly to drape over the tub flange. Some like to use a thin bead of caulk or sealant to seal or adhere the poly to the face of the flange.

With the poly in place, install your cement board. Read the cement board manufacturers installation instructions found online. I recommend you use screws to fasten the boards to the studs. There are special "high-low thread cement board screws" you'll find for sale, usually near the cement board. I recommend those. They are self-cutting, meaning that they create their own countersink in the cement board. They are coated so they will not rust. They are strong so they will not shear when screwing them in. Use a screw gun or a drill with a driver bit to install the screws. Drive the screws so the screw heads are just a bit below the surface of the cement board. You don't want the heads standing proud. You don't want to drive them too deep with the head sunk 1/4" into the board.

Use a damp sponge and wipe the walls down, removing any dust or debris.

Mesh tape. You'll find tolls of alkalai-resistant mesh tape in the tile section. It's slightly sticky. Tape the cement board joints.

Thinset. I recommend you use a modified thinset. Read the instructions on the bag. Mix a small batch. Let sit for 5-10 minutes. Mix again, then apply the thinset over the mesh tape, forcing it through the mesh and into the joint between the cement board panels. Then go over the joint with a taping knife, returning the face of the wall to essentially flat. You don't want to leave a speedbump of thinset on the joint. It's okay if you can see the mesh tape peeking through the thin veneer of thinset left on the joint. If you cover the tape with a thick application of thinset and create a speedbump over the joint, that bum will throw your tile installation off.

Walk away and let the thinset cure overnight.

Tile.

Grout the tile. Changes in plane (wall corners, tile-to-tub) do not get grouted.

Caulk the changes in plane.

Again, the above is a very basic overview for a beginner. It's not the only way. Techniques can certainly vary.