First, you have to answer me a question: Are you using resource pack plugins like ItemsAdder?
If no, you must install the resource pack at Minecraft client.
Open the game.
At main menu, click Options... button.
At options menu, click Resource Packs... button.
Zip plugin's /pack/ folder, then drag and drop it to this window.
Click the new resource pack to add it to game.
All is done, join your server and now you will see custom card textures!
If yes, you have to merge the two resource pack.
We use White_dye as material, if your other resource pack plugin does not use it as material, you just need copy and paste FlipCard's resource pack to the other plugin's resource pack.
If your other resource pack plugin use it White_dye as material, you have to merge the two resource pack. This is a little complex, maybe this site can help you. What's more, you can also contact our staff to help you merge!
FlipCard does not send the resource pack to players, so you need use the other resource pack plugin to send the merged resource pack to players.
Last: I really recommend you use ItemsAdder because FlipCard hooks into it and our staff also use it!