If you believe you have one of the Critical Illnesses covered by your policy you should contact the insurance company who will supply you with a claim form , if you are unable to request this for yourself somebody on your behalf should do this. The insurance company will then contact your doctor to confirm your condition and subject to any Critical Illness Cover Exclusions and you surviving the initial survival period will pay your claim. You will find contact numbers for claims within your policy documentation.
Once you have received your tax free payment you can use the money for whatever reason you want to pay off any outstanding debts or mortgages, pay for modifications needed to your house or car or pay for treatment or care.