The first step towards taking the C1 test will be to apply for provisional entitlement for the correct category, to do this you will need to complete a medical with your GP or other medical organisation and submit the forms (D2 and D4) along with your photocard licence to the DVLA. Once the DVLA has checked your application and returned your licence you can then take the next step.
As you are adding an additional category to your licence you will need to sit a theory and hazard perception test for that vehicle, in the same way that new drivers have to sit the theory test to drive a car.
Once you have successfully completed the theory and hazard perception test you will then be able to take your driving ability test.
If you are looking to gain a C1+E licence and already have a Category C1 licence you will only need to take a C1+E driving ability test as you have already completed the previous steps.