If you are planning your first installation, you should be aware that PVC liners are manufactured slightly under-size. This is to ensure a smooth, tight fit. (A baggy liner not only looks silly, but can cause your vaccuum to get stuck).
Liners are designed to stretch into position – so for optimum stretch, and easier installation, try and plan it for a warm, sunny day. The warmer the liner is, the more flexible it will be, and the easier the installation.
You should not try and install a liner on a day colder than 15°C. If you are installing in cooler temperatures (15°C – 19°C), and there is not a lot of sunshine, you should store the boxed liner in a heated part of the house, to ensure the liner is warmed through before starting.
Trying to work with a cold liner, without direct sun will result in a difficult installation, and poor end result.