Remove the grip from the French horn. Take the strap to a sink to avoid making a mess of your home. Most grips have Velcro or strings that are used to tie the strap to the horn. If your grip has string, remove it completely to avoid lubrication, which reduces its ability to hold a knot.
Apply 2 ounces of leather conditioner to your hands and begin to work it into the grip. Avoid getting any conditioner on the backside of the grip. If you condition the backside, the grip may lose some of its ability to stay in place and slip during performance.
Massage the oil into the grip with a kneading motion. Let the grip dry for a few hours. Then, with clean hands, rub the outside of the leather grip. Complete this process daily until the grip has been broken in.