!!!Back up your game files before trying this yourself!!!
With this guide you'll be able to change most aspects of a card's text , mainly the name of the card and the flavour text, this, combined, with modding the voice files and art, allows you to effectively create a new card entirely.
A zip/rar file extractor.
A text file editor, any will do.
If modding the art you'll need UABE and an image editor like Photoshop
If modding the voice files you'll need everything listed in the prerequisites of this guide
In GwentGwent_DataStreamingAssets, find and copy the 'data_definitions' file and paste it in a new folder outside the game folder.
Ctrl+F and search for the name of the card you want to change. You can freely change it, in my example I changed Vanadáin's card to Riordain. The flavour text is above the name, you can also freely change this text. When finished editing the text, save the file and change the file extension back to .csv.