Bathroom remodel, especially a new tub where do we start?

We also need a double vanity in this bathroom.

Turn off water to that room and start the demo. Take measurements and order what will fit in your space. Give your plumber the specs for replumbing where needed. Update your wiring, plugs lighting and exhaust. Put up the new walls: concrete board or fiberglass backer board in tub area. Fill in screw holes and seams with mortar not drywall mud. Put green board up for rest of bath. Make sure floor is strong enough for tile, 1 " plywood plus 1/2 " concrete board. Put in tub, tile, then vanity, paint, then toilet.

Steel or fiberglass tub removal you can use a sledge hammer or reciprocating saw (Sawzall) to break/cut it into manageable pieces. Turn water off at sink and disconnect plumbing. Remove old vanity top and vanity. Turn water off to toilet and remove. Cover toilet drain with cardboard. Remove old flooring. If redoing walls, strip down to studs. Insulate walls with fiberglass insulation. Install new tub. Put up cement board on walls around tub area. Drywall around rest of room. Put down cement board on floor. Place vanity/sink in location. Connect plumbing. Tile around tub area. Then tile floor. Finally install new toilet. Don't forget the new wax ring. Make plumbing connection to toilet. Prime and paint walls. Add in personal decor.