You can submit an application by completing the IDR Stage 2 application form which was issued with your IDR Stage 1 determination. You should include all relevant information and say why you disagree with the Stage 1 decision and how you would like your dispute to be resolved.
When you have completed the form you should send it to the following address along with all supporting documentation:
MyCSP
PO BOX 2017
Liverpool
L69 2BU
MyCSP will forward your application to Cabinet Office with the relevant supporting information.