Shower curtains protect the rest of your bathroom from water spray when you take a shower. They also allow easy access to the tub and shower stall when cleaning. Unfortunately, one of the trade-offs for this convenience is that the curtain doesn't always close the entire gap between it and the wall, allowing water to leak out.
Ensure that you have at least a shower curtain liner, if not the curtain itself, inside the tub or shower stall. If the curtain hangs outside the tub or the barrier at the bottom of the stall, it won't keep water inside.
Turn on the shower and look at how the water flows out of the showerhead. Look for any stray water spraying to the side. If you don't see any, skip Steps 3 and 4 and proceed to Step 5.
Remove the showerhead and look inside for scale. You can unscrew some showerheads by hand, but for others you'll need to use a wrench to remove the entire showerhead from the pipe. Protect the nut from the wrench with a cloth.
Dump out any scale you see and rinse out the showerhead. Hard water scale can sometimes collect inside the showerhead, blocking the holes that lead outside. This can cause streams of water to shoot out to the side. Reattach the showerhead when done.
Turn on the shower again and close the shower curtain just as you would if you were inside the shower. Look for areas where water is leaking out and see if there is a gap between the shower curtain and the shower enclosure that is allowing water to escape.
Attach one side of the hook-and-loop fastener to the outside edge of the curtain, a few inches up from where you found the gap that let water leak out. Attach the other half to the shower wall at the same height.
Close the curtain and fasten the two halves. Turn on the shower and check if the water still leaks out. Adjust the fasteners or add more if needed.