Fill a small plastic pot with a mixture of garden soil and mildly acidic compost with a pH no lower than 5.5. Pour 1 or 2 cups of water into the pot to saturate the soil. Let it drain for approximately 15 minutes before you pot the strawberry runner.
Locate a long, well-developed runner near the edge of a healthy and mature strawberry plant. Gently untangle the runner from the stems and leaves so it hangs free from the plant.
Set the prepared pot next to the strawberry plant. Pull the runner over the pot, and lay it across the surface of the soil.
Bury a 2-inch portion of the center of the runner in the pot of soil. Bury it 1/4 to 1/2 inch deep. Secure the runner under the soil by placing a small stone on top or by pinning the runner with a hog ring, industrial staple, or a bent piece of wire.
Water the pot with the runner whenever the soil feels dry at a depth of 1/2 inch. Keep the soil slightly damp but not saturated during the rooting process.
Check for roots in 30 days. Remove the stone or pin from the runner. Gently brush away the soil from the top of the runner and look for small, white filament roots near the surface. Rebury the runner once rooting is ascertained.
Sever the strawberry runner from the parent plant with a pair of pruning shears. Cut the runner flush with the soil.
Place the rooted strawberry runner outdoors in a partially shaded location with good air circulation. Keep the runner potted for two weeks, then plant it in a strawberry pot or permanent bed.