Step 1: Open up your card scan.
Step 2: Choose your paths tool.
Step 3: Go to View > Zoom > 800%
Step 4: Start clicking around the image, it'll make lines like this:
Step 5: Once you've gone all around the image make sure that there's a bit of space. It should look like this:
Step 6: Go to Select > From Path.
Step 7: Copy and Paste it, in your layers box right click on the floating layer, then choose "new layer".
Step 8: Delete the original layer.
Step 9: Save as .png