Measure the space for your walk-in shower carefully to determine the exact dimensions you are going to use to build it. Also decide upon a design, such as whether or not the shower will have a bench seat in it or not. If you plan on installing a bench, Tim Carter of the Ask the Builder website recommends a minimum shower width of 30 inches and a length of 48 to 54 inches to allow enough room for a seat.
Clear out the existing space if you're removing a tub or other bathroom components such as the sink or toilet. These need to be out of the way before proceeding.
Divide the entry way of the shower into two halves and mark a spot on the floor where the halfway mark is. Draw a 4-by-4 square around the mark with mark at the exact center of the square. Drill a hole at this halfway point to serve as a pilot hole. If you're working on a concrete slab, you will have to take a slightly different approach.
Cut the 4-by-4 hole using a jigsaw. Measure and cut a 48-inch 4-by-4 beam to insert into the hole. You will have crawl under the house to attach the beam to a floor joist. Do this, leaving approximately 36 inches of the beam exposed and sticking out of the floor. Attach the beam to the joist using self-tapping wood screws. Take small level with you when you go under the house. Try to ensure that the beam sits at 90 degrees or as close as possible.
Measure the distance from your 4-by-4 post to the back and front wall of your shower. Cut two 2-by-4s to lay on the floor between the post and the wall. Nail these to the wood floor. If you have a concrete slab, drill holes using a masonry bolt and bolt down the boards using masonry bolts. The 2-by-4 should buttress up against the 4-by-4.
Measure the distance from the top of the 2-by-4 on the floor to the top of the 4-by-4. Cut two more 2-by-4 inch boards this length. At the halfway point between the 4-by-4 and the back wall of the shower, nail one 2-by-4 so that stands straight up and parallel to the 4-by-4. Nail the other 2-by-4 in the same way, but against the back wall.
Measure and cut one additional 2-by-4 to sit on top of the 2-by-4s and the 4-by-4. Drive 3-inch galvanized steel nails through the 2-by-4 down into the ends of the other boards.
Cut two 2-by-8 inch boards to fit between the two 2-by-4s and between the middle 2-by-4 and the 4-by-4. Place wood glue on the end of each of these and then screw them together using 3-inch self-tapping wood screws. These pieces should sit on their side and on top of the 2-by-4 that you nailed to the floor. They will serve as a support for the rest of the frame and an additional lip at the bottom of the shower.
Measure the distance between the 4-by-4 and the front wall of the shower. Cut two more 2-by-4 boards to lay flat on the space where shower entry will be. Glue and nail these boards to the other 2-by-4. Nail one in place and then place the second on top of the first. Also drive nails diagonally through the 2-by-4s into the front shower wall stud and into the 4-by-by. These boards will serve as the lip for the shower door and will help solidify the rest of the frame.
Finish off your shower with the covering of your choice, whether it be a cut-to-fit fabricated shower stall or tile. If you will be adding tile, you will need to take some additional steps to provide a surface for the shower tile, otherwise your shower frame will be complete.