Wait until after the first frost of the fall or winter. The frost brings the sloes to full maturity and helps split the berries, which makes them easier to process for making gin or preserves.
Put on gardening gloves so you do not stain your skin with the juice. Wear old clothes.
Grasp the berries between your thumb and forefinger. Twist and pull to remove the sloes from the stem. Put them in a basket or bushel. You will need about 1 1/2 lbs. to make two bottles of sloe gin. If you are making preserves, pick enough to satisfy your recipe.