To prepare a standard curve for the particular enzyme first you need to check the activity of your enzyme sample at the optimum conditions (pH, temp.) using a reference method (e.g. reducing sugar assay Nelson Somogyi or DNSA) and a relevant polysaccharide as a substrate (wheat arabinoxylan, xylan).
Having this information you analyse a series of dilutions of this enzyme with the Azo-Xylan (Birchwood) at the same conditions as your reference method. The absorbances received for the dilutions used should be between 0.1 and 1.0.
Knowing the activity of the enzyme and dilutions used for the assay with S-AXBP, as well as having absorbance values for assay with S-AXBP, you can create a standard curve that can be used for determination of enzyme activity in further samples of the same enzyme.