You don't. Go over to Solaceon Town, and go to the cave on the left of the town. If you go through the cave, which are actually the Solaceon Ruins, until you see a little boy, who will tell you to go over his house and he'll give you seals in the shape of letters, as long as you show him an Unown you caught. If you go through the ruins, you will find Unowns throughout the whole Ruins. Find his house, show him your Unown, and he'll give you a seal in the shape of a letter. You need to catch a lot of Unown to get a lot of alphabet seals. If you buy a whole bunch of Quick balls, that might make them easier to catch.