When you finish a Long Rest, you can use your Crafting Tools and materials worth 50+ GP to improve a suit of armor into a Masterwork version. While wearing this armor, you gain a bonus to Armor Class equal to half your Masterwork Bonus, round up. You can't use this feature to create Masterwork Shields. If the armor already gains such a bonus, like a +2 Half Plate, you choose which bonus to use; you can't use more than one.
Other Creatures Wearing Masterwork Armor
Other creatures wearing Masterwork armor gain a +1 bonus to Armor Class. This bonus doesn't improve as you gain Craftsman levels.