Use the tape measure to measure the depth of your tub from the front to the back, as well as the height of the tub from the floor to the bottom edge of the tub where it will rest on top of the wooden frame. The length should already be included in the built-in cubby that was framed into your bathroom, but if you do not have a cubby you can measure the length as well.
Cut down your pieces of wood to build the frame. Use a miter saw to cut the 2-by-4 material down to size. Create four pieces that are to the length of the cubby (or the tub), as well as four pieces that are cut down to the depth of the tub. Also cut vertical pieces down to the height of the tub, making enough to fit around the entire perimeter. They are placed vertically ever 16 inches, so cut an appropriate amount based on your tub’s size.
Build the sections. Screw the vertical supports with the 3-inch wood screws, screwing through the bottom and tops of the frames. Build the front with two of the pieces cut to the length of the tub and sufficient vertical pieces, then repeat the process with the back and the two sides. Construct all four sections separately.
Install the sections. Place the back section against the wall framing for the home and screw it down into the floor through the bottom framing, as well as diagonally into the wall studs where appropriate. Attach the two side pieces in a similar fashion, building out the depth of the tub. Also screw these into the back section where applicable. Add the front section last, screwing it down into the floor and into the two side pieces to finish the frame.