1. First of all, buy a Book of Summons (Jania, at Annakol North Road, near the Feisty Chicken Inn, sells one)
2. Make sure the custom summons are enabled in the book of summons. You would be surprised of how many people would forget that back in the day.
3. Bookmark an empty slot in your book
4. See the level number in the summon name (eg.: White Tiger, level 5)? You must cast on the ground, near the target (NOT directly on it) the summon creature spell of the level of the creature or higher to capture (to capture said white tiger level 5, you must cast Summon Creature V or higher). Any