They should go to as far as you can get them in (at least 80%), but it doesn't have to go to the very end. Use a twisting motion to slide the flange into the ring collar.
Be sure to use some water spread around the end of the flange to help it go on more easily. Most moms just lick their finger and rub it around the silicone at that area (it doesn't touch milk). Don't use lubrication like coconut oil in this area because it's too slick and the silicone might slide out.
Reviewed by Diana West, IBCLC
June 10, 2021