I would look at a luggage tag pattern. Or just make your own.
Trace the ID card on paper or card stock twice. On the first one measure 1/4 inch around the whole thing. On the second one.... measure at least 1/4 inch around 3 sides (short, long, long) and add 1/2 inch at the final short side (for the clip hole) Use a small bottle cap or something similar to round the corners of your drawing. On the first piece, measure in and mark a line at least 1/4" from your original traced line to make the "window"
Cut a prototype from scrap leather, an old pair of jeans, or what ever you have. Then punch and sew it up. If the ID card fits well. Make one from your "good" leather.
Another way is to use a drawing program and draw one up. I think it is good to learn both ways to make patterns.