Put your new card textures to /plugins/FlipCard/texture/card folder.
Use /fc reload command to reload the plugin.
Open config.yml, you will find new textures configuration have been generated. You can edit its configurations. Use /fc reload command again if you have edited config.yml here.
Open /plugins/FlipCard/pack/ folder, zip this folder and this is the new resource pack.
Add new edge texture:
Put your new edge textures to /plugins/FlipCard/texture/edge folder.
For new card or edge texture: Just type /fc reload command.
For modify generated card or edge texture: You need first delete those saved config in config.yml file then use the reload command. The config should be like this:
Configs Pack
Use custom font to add your UI texture to the game, I can give an example pack here: