While inserting the disc, using your thumb can be super helpful to get the disc down and back into the right position. Many users find the thumb naturally goes in that direction and is steadier than an index finger.
Simply pinch the disc and insert it down and back into the vaginal canal. Once you can no longer keep the disc pinched, place your thumb on the rim and push the disc all the way back (angled down towards your tailbone) the rest of the way until it's just behind the pubic bone. You may need to insert another finger afterwards to pull the rim of the disc up behind your pubic bone; doing so will help keep the disc in place.