LogFAQs > #973183788

Now that the protagonist, Lann, and his sister, Reynn, got brought to the tutorial forest, its time to learn about how to capture the monsters of this game which are called mirages. How to capture them is...well, this serves as the basic gist of it.
https://gamefaqs.gamespot.com/a/user_image/2/0/9/AAXh4NAAEbvh.jpg

Okay, maybe not everything. There's the matter of how to imprism any given mirage. That because unlike pokemon that's a simple 'Get their HP as low as you can and maybe put a status on them for good measure', every type of mirage has their own requirements on how to catch them. Which I find to be slightly more engaging way to capture monsters as you have to pay attention to what they're like. Case in point
https://gamefaqs.gamespot.com/a/user_image/2/1/0/AAXh4NAAEbvi.jpg
This one needs to be physically attacked to enter that state, which you can do with just a simple plain attack. Do so, and they start glowing.
https://gamefaqs.gamespot.com/a/user_image/2/1/1/AAXh4NAAEbvj.jpg
In that state, you can try to capture them if you have the right kind of prism on hand. It might not work the first time, so you may need to repeat the attempt but you won't waste the prism if it does fail so you can keep trying.

And once you capture one (or a few of them), what do you do with them then? Simple, you have to add them into your battle party by sorting them into your stacks. ...What's a stack, you ask? Well the simple version is that you can stack mirages onto each of the player characters to let them all share the same stats, abilities, and resistances and weaknesses. For example:
https://gamefaqs.gamespot.com/a/user_image/2/1/2/AAXh4NAAEbvk.jpg
This is Reynn's base stats, and she currently has no special abilities other than to imprism when that occurs. However if you want to stack the small mirage we just gained on top of her (stacks always have to follow a large, medium, and small pattern from bottom to top. You can't have, say, two large units at the bottom followed by a small one on top) then her stats would now look like this.
https://gamefaqs.gamespot.com/a/user_image/2/1/3/AAXh4NAAEbvl.jpg
Of note, her HP, AP, Strength, and Magic stats are all greatly larger in this setup as the two add their stats together. And Reynn's stack would now be able to use the Ram attack that the chocochick possesses. In short, you really should learn how make good stacks in order to get far in this game.