Consider practicing with play dough before breaking out the sugru.
Here are the steps to make a simple business card holder.
1. Roll a small piece of sugru between your fingers into a tube shape. I used a little less then half a packet to make mine.
2. Push it against a clean table top to give it a flat bottom
3. Insert a card to create the slot. Give the slot a slight back angle so that the card slightly faces up. This should not be a card that you care about. Mine got a slight blue stain from the sugru when it was wet.
4. Wait for it to dry.
You may need to insert the card several times to get the spacing and angle desired.