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It might have fit better in the Community Questions & Feedback Signature Thread, however this is relatively simple to answer. You can either make a Pokémon Trainer Card, which you can do so here (https://pokecharms.com/trainercards/) or you can use sprites of your team members to place in your signature. I find this site best for doing so (https://projectpokemon.org/imagedex/index.php?type=pokemon&natid=1). That's really all there is to it.