How do I apply for 3C Leave?

Hi all
 I applied for my FLR M visa a few days ago and I have biometrics tomorrow.
My current visa (Appenedix FM fiance') ends on Nov 16 this year.
I have this website and I understand what 3C leave is: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/953205/3C-3D-leave-v10.0ext.pdf
But I have no idea how to secure 3C leave.
Do I fill out a form?
Is there a charge?
 Do I need to do biometrics again to receive it?
 I have no concept of how to do any of this.
Thanks to anyone who can explain it to me! 
Kathy

I'm starting to think it is something that happens automatically once you apply for the next visa, in my case,  FLR M

You don't 'apply' for 3C Leave, it's just something that happens automatically... it's an automatic extension of your current visa if your FLR(M) hasn't been issued before November 16th.

As long as you submit your new FLR(M) visa application BEFORE your current visa expires, if your new visa hasn't been issued by the expiry date, then the 3C Leave will kick in automatically when your visa expires. And as long you do not leave the UK while waiting for the new visa, your 3C Leave will remain valid until your new visa is issued (leaving the UK invalidates 3C Leave).

So, as an example, if you applied for FLR(M) in late September 2021 and your current visa expires on 16th November 2021:

Scenario 1:
- Your FLR(M) is issued before 16th November 2021... so the FLR(M) simply replaces your current, still-valid visa, and 3C Leave does not come into effect

Scenario 2:
- Your FLR(M) takes longer to process and has not been issued by 16th November 2021
- On 17th November 2021, 3C Leave kicks in, automatically extending your current visa while the FLR(M) is being processed
- Your FLR(M) is issued on 12th December 2021, so your 3C Leave automatically ends on 12th December and is replaced by the FLR(M)

Thanks so much
After looking over it again today, I was beginning to think this was the case! 
Thanks again!