To register for the Life in the UK test, first go to the government website: https://www.gov.uk/life-in-the-uk-test. There you can find all the instructions on filling in the application and a link for registration: https://www.lituktestbooking.co.uk/lituk-web/. When registering for the test, you will need the following information: your email, debit or credit card, and your ID.
If you have any issues, you need to contact the customer support service. The phone number can be found on the government website: 0800-015-4245.