Custom chapter questions

Hey all!

Apologies if this has been posted a million times, I have read a few posts already but I just had a few questions.

I’m very new to Warhammer 40k (have been reading into it for about 2 months now) and had some questions regarding creating my own chapter and how I can keep it within the 40k lore.

I really want to make an offshoot of the iron hands as from what I’ve read, the real focus on stoicism and standing my ground in battle is what I want my chapter to be focused on.

Is there a basis I can go off when creating this, are there rules I should adhere to when crafting the lore?

If there are some resources online that I can read through or great videos explaining this, that would be awesome to look into!

Cheers 😄