No matter how well you spread your liner out when you set your pool up, it will almost always develop small wrinkles as you begin to fill the pool with water. These wrinkles must be worked out of the liner before the water level gets too deep. Once the level has reached a few inches, the weight of the water makes it nearly impossible to get any more wrinkles out. On the other hand, if you have no water in the pool the wrinkles never seem to go away.
The trick is begin filling the pool with water (a regular garden hose is fine, more than one will fill more quickly), then get into the pool after the level reaches about one inch. Leave the water on, and get down to your knees and work the wrinkles out by hand. Work from the center of the pool outwards towards the edges. Keep one hand close to you, flat on the ground to stabilize the liner, then use the other hand to “pinch” the liner an pull it outwards towards the edge of the pool. Again, work from the center to the outside edge, then move back to the center and repeat, moving out in a different direction each time. Keep working around the circle of your pool. As more water is added to the pool, your movements will result in fewer and fewer wrinkles. If you’re still dealing with large wrinkles and they’re getting to be very difficult to work out, then you may have too much water. Turn the water off and remove some by siphoning or with a bucket. It can also be helpful to have a partner outside of the pool to stretch out the last few feet around the outside edges.
Using these techniques, you’ll find that the wrinkles seem to get smaller and smaller, and eventually you will end up with a smooth bottom that’s almost completely free of wrinkles. Turn the water back up, and when the level has risen another inch or two, step out of the pool and then let it fill the rest of the way.