Wait for at least six months after getting your cartilage pierced before you begin the process of stretching it. This is the least amount of time required for the previous piercing to heal. If you try to stretch your cartilage piercings quickly, it is likely to form hypertrophic scars and make things really messy.
Once your piercings are six months old, begin the process of stretching your cartilage piercing by taking a warm shower. Do not forget to take off the jewellery before getting into warm water.
After you have taken the bath, apply some lube on the piercing, as well as the jewellery that you are about to insert in it. Make sure the jewellery is not too big.
Gently slide the jewellery through the piercing. It is normal to feel a bit of resistance. Just remember to be slow and gentle. If the cartilage has healed from the piercing and is ready to stretch, it will allow the jewellery to slide through. Try not to bump the jewellery into the cartilage, as that will surely hurt.
Once the entire piece of jewellery has slid through, stick with it for the next six months or so. Avoid sleeping on this side during this time.