Here's some basic instructions to making a cardback.

The front and backs are just printed onto A4 Glossy labels (100g).

Images should be printed at 15.1cm x 22.8cm

Also shown is a plain piece of card (160g), vintage C3-PO and sack and the plastic bubble

Cut out the cardback and stick it onto the cardboard.
Now cut the card around the sticker
Next, cut out the opposite side from it's sticker, place on the blank side of the card and trim to the edge.
Now, using just a small amount of clear glue (I used about 4 drops of Super Glue) place the figure on the card and stick the bubble over it.

I used some superglue for this and only a couple of small drops on the edge of the bubble

Here's the custom made C3-PO next to a real carded figure. Not a bad result.