Write a protocol describing the steps you are going to validate (specificity, linearity, accuracy, precision...) that is compliant with ICH Q2(r1). Once the results have been reviewed write a final report, and an SOP initiating the new method. Be sure ALL documentation is signed!
You have to compare your data with standard protocol for the validation of analytical method such as by analyzing certified reference materials (CRM) in order to check the the accuracy of the method.