You're frame is built, now what?
The next step is to put the liner down. However, there are a number of thing to consider before taking this step.
How reliable is your source of water? If you're going to use your garden hose to fill your rink, then I suggest waiting until the last possible moment to put your liner down. There are a number of things that can go wrong if you put it down early. The wind can rip it. Leaves will fall into it. Leaves must be raked, without damaging the liner, out of the rink before filling it with water. More disscussion on leaves can be found here.
On the other hand, if you don't have a reliable water source, or you're worried about over using your pump, you can put the liner down in the fall and hope to accumlate some rain water. I have heard that this can be somewhat effective, but you still have to remove leaves.
How are you going to secure your liner. Until there is water in your liner it is in danger of blowing away. Even after water is securing the main portion, the edges will still blow back into the rink. I recommend either rocks or a staple gun. If you use rocks you don't have to worry about stapling the liner without enough slack. If you do not leave enough slack with staples the liner will rip and cause problems. Rocks will simple slide over the liner and not rip it, but they're somewhat more difficult to round up and place.
What's the weather like? Hopefully you're taking my advice and waiting until the last possible day to put your liner in place. But what is the best day for this? First of all, the weather needs to cooperate. You're going to need 3 solidly cold days in a row. Ideally you would put your liner in the day before a spell of days with highs at most in the upper twenties and lows in the teens. It will work if you're bordering on freezing during the day and only getting down to 25 at night, but it will take longer. Much of this is dependent upon how much water you need in your rink. More discussion on that later. Low wind while you're putting in the liner is also useful.
Ok, you're now ready to place the liner.
The rink has been designed so that your liner will cover it with one piece. The 20' width is about 3 feet wider than the floor of the rink. This extra 1 1/2 feet on either side is necessary to go up and over the rink boards.
Cut your liner at about 40', or about 4' longer than your rink from end to end. Like the side boards, the liner will need to go over the end boards.
Secure your liner in place. Put rocks heavy enough to withstand the wind on top of your liner on the outside of your boards. Or, if using a staple gun, staple your liner to your boards on the outside of your boards leaving plenty of slack for the liner to move as you fill the rink.
One trick is to push the liner between the rebar supports and your boards. This helps keep the liner in place while you secure it.
Time to fill your rink!