Repairing your weapons works a little differently in Cindered Shadows than in the main Fire Emblem: Three Houses Country. To repair your weapons, you will need gold and blacksmith stones. You can also use Smithing Stones to forge weapons at the blacksmith's.
Every time you complete a battle you will get 20 Smithing Stones, which allows you to easily repair weapons whose durability has been reduced during combat. To repair your weapons, head to the blacksmith who is indicated on the map by a hammer icon. Interact with it and select Repair, then choose the character whose weapon you want to repair. Click on the weapon and the bottom of the screen will show you the cost in Smithing Stones and Gold. This will vary from weapon to weapon, depending on durability lost and weapon type.
The only weapon you don't need to repair this way is Byleth's Creator Sword. This will automatically regain 20 charges after each battle. You won't need to repair all of your weapons after each battle, just the ones that have taken too many hits to their durability to be useful in the next fight.
If you find any rusty weapons, you can also use Smithing Stones at the Blacksmith to upgrade them and bring them back into combat condition. You can find a Rusty Sword in the store where Ashe explores at the start of the game.
Remember, while the Cindered Shadows DLC contains a campaign that you play separately from the main game, you can still recruit the Ashen Wolves in your other parts of Fire Emblem: Three Houses.
