Due to some comments, I am remaking this instructable, I hope this one is more clear and easier to understand. I have also taken screenshots of the project rather than pictures from my phone. So, here we go.
Step 1: Base
Dig a 4x3 rectangle. Now dig those same dimensions down one more so it looks like the 2-block-deep hole in the picture. Now, using your choice of materials, fill the bottom of the hole. Now, fill the layer above the one you just placed down but leave a hole like in the last picture.
Step 2: Walls
Now, build a wall 5 blocks up on the back portion of the shower (see picture 1). Do the same for the other two walls like in pictures 2 and 3.
Step 3: Roof
Now fill the roof in. Add in two blocks so that there is an indent in the roof that will be used later.
Step 4: The Piston
Punch out a hole on either side of the wall. It should be 3 blocks up from the floor, or one block taller than your character. Now put the piston in the hole and make sure it is facing in.
Step 5: The Water
That indent you created in the last step will be used to place the water
in. Put it up there and watch it flow down. There will be no current due to the hole you created in step 1.
Step 6: Lever
Put the lever right next to the piston. Now you can control the water flow. Simple as that.
Step 7: Door
When you stand back from your project, you will notice that there are 3 blocks left in the opening. Fill in the top block and place a door into the remaining two.
Step 8: Finished Product
Now you have a shower in which you can control the flow to. I hope this is better than it was before and that it is easier to understand. I love hearing from you guys so keep commenting :)