Prune back any new growth that the layer has produced during the season. Remove the black plastic and gently loosen the roots and moss; then cut off the stem with sterile secateurs below the roots.
Place the layer in a pot filled with moist soilless potting or cutting mix, and put it in a protected area of your garden until the first hard frost of autumn. Once the layer is dormant and the leaves have dropped, store the pot in an unheated garage or porch. Do not water the layer until the following spring, when it should be gradually acclimatized to outdoor conditions.
Plant out in the garden in late spring or early summer, and enjoy!