Scraps 
are used to create new cards for a player's deck.
They can be aquired by:
- Playing and winning against a Non-NPC-Player
- Milling (destroying) cards already in a a player's possession.
- Getting a GoodGame (GG) from your opponent can grant you either 5 Scraps or 5 Ore
- Through certain nodes in the Reward Trees
To craft or mill a card, navigate to the Cards menu, then the Collections menu. Once in collections, click the desired card - greyed out or not - and then click the craft or mill button at the bottom of the screen.
The amount of scraps gained from milling is extremely low compared to how much they cost to craft.
Note: Crafting / Milling cards is disabled before level 3.