You will some answers to your question at the following previous question:

https://www.researchgate.net/post/Can_anybody_suggest_method_determination_of_cellulose_content_in_seaweed_samples

The paper of Laurens et al. 2012. (Anal. Chem. 2012, 84, 1879−1887) is also interesting.

I think you can follow the method described by Van Soest

Thank-you. I have tried simliar methods without sucess but this gives me more information to work out the method.