To store a waterproof sofa cover for a long time, follow these steps:

Clean the cover: Before storing the cover, ensure it is clean and dry. Clean the cover as per the manufacturer's instructions, and allow it to dry completely.

Fold the cover: Fold the cover neatly, making sure to avoid any creases or folds that could cause damage over time.

Place the cover in a storage bag: Use a storage bag or container that is large enough to accommodate the cover without causing any compression. Choose a container that is water-resistant or waterproof, as this will help to protect the cover from moisture, dust, and insects.

Add moisture-absorbing packets: Include some moisture-absorbing packets or silica gel packets in the storage container to prevent any moisture build-up.

Store in a dry, cool place: Store the container in a dry, cool place that is free from direct sunlight and extreme temperatures. Avoid storing the cover in an attic or basement, as these areas are often damp and prone to temperature fluctuations.

By following these steps, you can ensure that your waterproof sofa cover is stored properly and will be in good condition when you need it next.