Repairing your weapons works a little differently in Cindered Shadows than the main Fire Emblem: Three Houses campaign. To repair your weapons, you will need Gold and Smithing Stones. You can also use Smithing Stones to forge weapons at the Blacksmith.
Each time you finish a battle, you will get 20Smithing Stones, giving you an easy way to repair weapons that had their durability reduced during the fight. To repair your weapons, head for the Blacksmith, who is denoted on the map by a hammer icon. Interact with her and select Repair, then choose the character whose weapon you wish to repair. Click on the weapon, and the bottom of the screen will show you the cost in Smithing Stones and Gold. This will differ from weapon to weapon, depending on how much durability it has lost, and the type of weapon.
The only weapon you don’t need to repair in this way is Byleth’s Sword of the Creator. This will automatically regain 20 charges after each battle. You won’t need to repair all 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 Rusted Weapons, you can also use Smithing Stones at the Blacksmith to upgrade them and get them back into fighting condition. You can find a Rusted Sword in the storeroom where Ashe is exploring at the start of the game.
Don’t forget, while the Cindered Shadows DLC contains a campaign that you play separately to the main game, you can still recruit the Ashen Wolves in your other playthroughs of Fire Emblem: Three Houses.