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 source will do: directly by yourself, scrolls, items, you name it. (also, the creature has a reflex save to not be captured. the DC is your character level, iirc)
5. If all goes well, eventually the monster will be captured and readily avaliable to use, rename and what not. To summon your creature, cast the Summon Creature spell, but this time, the level must be equal to the monster level. (again, in the White Tiger, level 5 example, you MUST use Summon Creature V. This time VI and above won't do.)

There are many custom summons to capture and they range from level 1 to level 9 summons. In most cases, what you can capture is far more powerful than the default summons. There are areas known to spawn such capturable creatures - talking to players is helpful - either in game or on Discord.

This system was down when Annakolia moved to Beamdog EE since Windows NWNX did not work with EE. We have made adjustments to fix this peoblem and our custom capturable summons are once again collectable in your summons book and then able to be spawned instead of the default summons.

Since there are a variety of each level, one can search and collect - custom name and choose which one to summons for any situation.