The ICC is only issued by the RYA out of the UK. In order to gain an ICC you must either:
(a) Pass the RYA Day Skipper theory and practical and gain the RYA Day Skipper Certification. Once you have done this you can download the application form inside the NauticEd RYA Day skipper theory course. Fill it out and send in the application to the RYA.
(b) take an ICC assessment with an authorized RYA school. This is a very rigorous assessment and if you have not done theory work for a while like tide calculations, chart plotting, lights, buoys, sounds, etc then more than likely you will fail the assessment. To brush up on the assessment theory work you can take either the RYA Day Skipper course online with NauticEd or take the NauticEd Bareboat Charter Master theory courses. Both will prepare you adequately for the theory part of the assessment. When logged into NauticEd you can download the OTW (on the water) ICC/SLC/Bareboat charter master Assessment rubrics document. There you will find the document that the instructor uses to assess you. Once you pass the assessment, the RYA instructor should have an application form for you to fill out to gain your ICC.
If you live in North America, it is probably a simpler process to gain the NauticEd SLC license which serves the same purpose as the ICC. Both licenses are equally accepted as a valid sailing license for the Mediterranean and Seychelles waters.
To gain the NauticEd SLC see this link - https://sailing-blog.nauticed.org/how-to-gain-your-slc-mediterranean-sailing-license/