FLR(M) - Where to find application guidance / requirements?

[777GE90]

Hello, I recently had my wife join me in the UK after a successful Leave to Enter application (i.e. joining me as my spouse). I had a immigration specialist sort all this out for me so I don't know much about the process.

I am planning to do the next extenion application myself, I understand it is called FLR (M), i.e. further leave to remain (marriage). However, I wanted to find all the details required for the application and make sure I understand it correct and build up the necessary evidence over the next 2 years. The Gov website is very basic and doesn't really mention much about required documents and what not.

Does anyone know where I can find detailed guidance as to what is required? Things like how many different letters of evidence we need, whether she needs to work or not, how long we can go on holiday together, whether we need to supply photos and whatsapp messages again or not, etc.