Part 1: Introduction
First, you have to go to the right tab to make a trainer card. You can find it at the top menu bar, where it says "Trainer Card Maker"
The next page should be the TCM interface. It should look something like this, minus all the circles. Just put down whatever information you want under the red circles; the purple circle will have a preview of what your card looks like.
If you already have a friend code on your profile, you can import it into your card like so.
The next step is VERY IMPORTANT. Do you see the bit below the tabs? The bit that has stuff saying "Welcome", "Backgrounds", "Characters", etc? That's what you'll be using to edit your trainer card. Click on each category to edit your card.