There's plenty of highly detailed information here in the forum.
I do really suggest you devote some time to research, that way you can be better prepared to ask specific question around your own circumstances.

There are very many folks here who have followed the path you will take and will be happy to help you.

It's really a big ask to provide detailed information to generic questions.

May I kindly suggest you contact your local registry office in the first instance who will tell you exactly what they require.
The CNI you receive will be valid for 3 months.
You need to exchange the UK CNI for a local CNI and this needs to be done at the British Embassy. An appoinment needs to be booked quite some time in advance. Chaeck out the British Embassy website for the latest info on that and how to get your appointment.

You need to spend time to understand the processes and how they will impact your and your fiance(e) right up until after you are married.
Step by step