Instructions
Visit the Moomoo Farm along Route 39. You can get to the Moomoo Farm by heading either west from Ecruteak City or north from Olivine City.
Enter the barn attached to the side of the farmhouse. Inside, you will find two girls and a Miltank. When you talk to the girls, they will tell you their Miltank is sick, but it will get better if it eats some Oran berries. Oran berries can be obtained by trading shards to the spiky-haired boy in Violet City. Shards can be obtained by smashing breakable rocks with any Pokemon that has learned "Rock Smash."
Give the Miltank seven Oran berries to make it feel better. After you have healed the sick Miltank, talk to one of the girls again to receive the seal case.
Go south from Moomoo Farm and head to Olivine City. There are two houses just northeast of the Olivine City Poke Mart. Enter the eastern-most house and talk to the little girl inside to get three seals. As long as you have the seal case, you can visit the girl every day to get three new seals.
Apply the seals to your Ball Capsules. Visit the nearest Pokemon Center and access your in-game PC. Select "Ball Capsules" from the list of options, then choose the seals you want to use. Place the seals on any of the Ball Capsules you currently are carrying by dragging and dropping them with the Nintendo DS stylus.