Short explanation by W1ndows: Card game RNG changes based on what happens in battles. Which enemies you get in world map, what you choose to do in the battles, what the enemies do, and the camera angles at the end of battles all affect the RNG differently. On top of that, the Card RNG advances by 1 each frame that you wait in the "Play" screen before a card game. This, accompanied with the information on what RNG Zell card is played, lets us manipulate the cards by waiting in the "Play" screen.

Research by http://www.speedrun.com/user/pingval : http://pingval.g1.xrea.com/psff8/research/index_en.html#zell-card

Has a lot of great information about FF8 in general, but also the Zell card manipulation. Pingval is the person who discovered this manipulation possibility and made the Ruby program that helps you time your "Play" screen correctly.

The tool FF8 Utilities (found in Resources section) is extremely helpful in the case that you don't want to play around with Ruby and its files. It also has a counter built in to make tracking the RNG easier. Very strongly recommended to use this, instead of Ruby etc. Tutorial for first try Zell card by https://www.speedrun.com/user/bramhallthefifth : (14min)

Fairly in-depth guide on how to get Zell card on the first try using the FF8 Utilities program. Has a small step count tutorial for the start of the game as well as an explanation on what to keep track of for the card game RNG.

In the case that you don't want to use this tool:

A pastebin by http://www.speedrun.com/user/luzbelheim that explains how to setup Ruby and get the script working: http://web.archive.org/web/20170320143720/http://pastebin.com/8FQRf3fk

The link to excel file that Luzbelheim has made, can be found in the pastebin also: https://mega.nz/#!s1QRhLqY!UqCOBxfTG5iuqbKvh5_vxqvmGhEVs9o-vhcu9oJPe74

Notes: If you use Pingval's cards for the early Quistis card, you must change the early_quistis value to "pingval" from "luzbelheim" (without quotation marks).

To use this Pingval's script on the PC version, you can find the appropriately edited Ruby file in the resources section. Has Pingval cards by default.