Fill a bowl with juniper berries. Crush the berries with a wooden spoon to the point where you split the berries, but do not pulverize them.
Pour 1 inch of water into a large stew pot. Set a cooling rack over the water; the rack should not touch the water and should fit inside the pot perfectly. Line the rack with cheesecloth and set a glass jar in the center of the rack.
Pour your crushed juniper berries into the pot around the jar, making sure none get into the jar. Spread them out evenly on the cheesecloth.
Set the stewpot on a large burner. Place a metal bowl on top of the stewpot, centering the bottom of the bowl over the glass jar. The jar should not touch the bottom of the bowl. Fill the bowl with ice.
Turn the heat on medium low. The water should begin to steam and push up through the berries, vaporizing the oils in the berries first. The vapor will condense against the bottom of the bowl, run down the bowl and drip into the jar. When the ice in the bowl turns to lukewarm water, your oil is done.