At its best:
Choose a sheltered spot that won't be disturbed. Dig a shallow trench the same size as the old growing bag. Lay the bag in the trench and gently firm the compost.
Cover the bag with grass clippings or plant prunings, ensuring no unsightly edges of the bag are on view. Frogs love to squeeze into tight spaces, so pack the clippings down.
Spread a thick layer of leaves over the top. If you've chosen a windy spot, add some twigs to hold the leaves in place. Leave the hibernaculum undisturbed until April.