I see from your previous posts, you have only recently entered the UK as a spouse on a 33-month spousal visa.
For FLR(M), all the documents you need are listed on the FLR(M) application form:
https://www.gov.uk/government/publications/apply-to-extend-stay-in-the-uk-as-a-partner-or-dependent-child-form-flrmIt's basically all the same financial and accommodation documents as for the spousal visa, just updated, but the relationship documents are slightly different.
This time, instead of evidence of communication during your relationship, you provide 6 documents addressed to both of you at the same UK address (either 6 joint, or 12 separate, or a combination of both, giving 6 in each name), from at least 3 different official sources, spread evenly over the last 2 years. If you haven't been here long enough to get 6 documents yet, then you need to write a letter explain this. All the details are on the FLR(M) application form.
So, if you have only just arrived in the UK, the things you need to do now, to start preparing for FLR(M) in 2.5 years are:
- get your name on as many official documents as you can (tenancy, council tax, utility bills, bank statements, NHS, etc.)
- turn OFF paperless for everything, so you both receive all your mail in the post
- start saving ALL your mail