We recommend cleaning the filter after each use of the bottle.
Our team has found it's best to start with a soft yellow sponge (preferably one with no rough green side) and wet it with water and an oil dissolving soap like Dawn or Palmolive. From there you take the sponge and gently wipe the mesh on the inside and outside to get the oils out. You don't need to use much pressure at all, it's more about using the soap to break up the oils.
Once it looks good and the oils are gone, you can give it a final rinse and set it out to air dry. After it's completely dry you can store it back in the cold brew bottle if it's empty (we don't recommend storing it in anything until it's totally dry, as leaving it wet and enclosed may cause mold to form).
If you find yourself needing a new filter, you can purchase one from Hario here.