I have installed debian and now need to add contrib and non-free to some file.
I have done this a few times before but now I have completely forgotten these simple steps.
I've trawled through my previous threads but can't find the right one.

Essentially, I need to add contrib and non-free to a file, then install linux non-free from the repos, install pavucontrol and change the soundcard setting (so my non-free radeon soundcard will work).
It's a bit trivial, but can someone please help?

You need to add their URLs to the file /etc/apt/sources. I think Debian Wiki is the goto for this sort of question.

you may want to add 'contrib' and 'non-free' to your /etc/apt/sources.list
e.g. - this is from my sources.list-file, using a mirror in germany:
than run 'sudo apt-get update'

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Oh please God, not the debian wiki!

Why not? It's a treasure trove of information. If you do a search on duckduckgo for a GNU/Linux related problem, I'd be surprised if among your first hits were not two sources:

- The Debian Wiki
- The Arch Wiki

Both are indispensable, and for the most part the information within both can be applied to any distro.

Related to the OP, typically on any new install I do a

The good news is that in the newest D-I it actually asks if one wants to enable non-free repos and doing this manually after installation is no longer required.

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Thank you, that is exactly what I was looking for!

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The Arch Wiki is certainly very well written.

Wow, this will stop immediate problems after a debian install onto most home desktops.
Who thought of that idea? I hope the person is still alive.

The default debian OS is configured for main and contrib on /etc/apt/sources.list.
So if you don't have an Intel cpu with further debian-friendly hardware - then you will experience a world of hurt.
I have even opened a thread asking what hardware I need to use debian out of the box - and no-one knew!

Of course, main, contrib and non-free are good principles. And they should be upheld.
But if I install debian, then I want it to work first and then ask questions later.

Because it's pretty much the same hardware as any other distribution. What you consider out of the box, others may not think it so big an issue of adding that repo and fitting the "out of the box" requirement. You had to add a repo, which is far better than the hardware not be supposrted all. To some here, that would fit into the category of not working out of the box.

There's a reason non-free is not enabled by default. It's a philosophical one, but one nonetheless. You're free to use any distro you like if you don't agree with their philosophy. There are other distros that impose different restrictions due to other reasons decided by the heads of that distro, it's unfortunately a part of the ecosystem you'll have to deal with as you use Linux.