Embroidery Library is a good place to look they info on all sorts of things and good sound advice is is too. I think open type designs would work best as a filled design has too many stitches and therefore too many holes around the egde so the work may 'pop' out of the card.
I have given the link to the home page just click on Projects on the lefthand side of the page.
Welcome to cute is a grand place to start any machine embroidery related query!
Also, make sure your design doesn't have any jump stitches that land somewhere it won't be covered. Otherwise, it will just leave a hole in the card where it can be seen.
Lucy What a great question. I've thought about it, but never tried it. Thanks for asking. And Welcome...
You don't hoop the card but the stabilizer. The emblibrary video is a good one. You'll find video's on all sort of embroidery on that site as well. Worth looking when you are starting something you never did before.
Here is another tutorial from Starbird:https://www.starbirdstockdesigns....