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 the disc 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 back and down the rest of the way until it's just behind the pubic bone. You can then user your finger to prop it up behind the pubic bone if needed. This will keep the disc in place.