To convert your tub to a walk-in bath with a cut-out door, draw the sections you wish to cut onto the surface of the tub, seal the bathroom, and use a saw to cut out the sections. Finally, caulk a bath door kit onto the tub.
Begin by taking measurements of the length of the tub. Make a mark on the tub at the halfway point by dividing the measurement in two and measuring from one wall to the center. Make a second and third mark 1 foot to the left and 1 foot to the right of the first. Draw two vertical lines from the left and right side-marks 10 inches down the tub. Draw a horizontal line from the bottom of one vertical line to the other.
Turn off all vents in the bathroom, block the opening of the bottom of the door, close any windows, and place rags inside the drain and faucet of the tub and sink. Place a respirator over your face to prevent inhaling any debris during the cutting process.
Use a reciprocating saw or jigsaw to cut into the tub, closely following the three penciled lines. Without handling the edges, remove the center piece of the tub. Carefully clean the cut edges of the tub. Use a caulking gun to add a thick layer of waterproof caulk to the outside of a bath door kit, and fix the kit onto the tub. Check the door again after 24 hours, and ensure there are no leaks or loose areas on the door or in the tub.