If it is only dust a breath of exhaled CO2 or some other source should do. Next a gentle brushing with a soft camel or sable brush.
I have cleaned a couple of really dirty ones with dish detergent and warm water. A thorough rinse afterward, while they are still wet to get rid of any film left by the detergent. Let them dry while standing on their edge,
I like microfiber cloths for regular glass filters and my lenses but never on Cokin as I don't know if the cloth and plastic filter would create static electricity.
The Cokin filters should clean up Ok. they are Cast Acrylic plastic, and optical grade material, wash as suggested and dry with chamois leather to stop spots. Most are special effect types and minor scratches will not matter anyway. The microfibre cloth should work OK, but static does build up on Cokin anyway, far for than glass. The popularity was based on the graded colours and the soft focus types, including spot types, but digital and photoshop has rather taken away the market! The advantage of the Cokin spot and graduated types was being able to move the effect anywhere in the film cameras frame. Stephen.