Lessons from this one (and from this Netflix's piece) helped us to throw out ~15 bags of clutter right before our kiddo was born. There's some metaphysical BS though. Filter it out and you'll find this book a remarkable piece.
[!tip] Tip According to the author it should be enough to do it once in a lifetime, but my experience is that it's good to do it every now and then (like during the moving out, after a kid is born etc.).
Declutter the whole house at once. Occasional tidying up won't change your relationship with possessions and the clutter will find its way back into your life.
Go through your possessions by category - items belonging to one category may lie in different locations. Put them in one place (e.g. on bed) to see how much of them you have.
Grab every item and decide if you want to keep it. Focus on its purpose, why you have it and if it brings you joy. If it doesn't serve any purpose and you don't need it anymore, remove it from your life immediately.
Don't listen to music or watch Netflix in the meantime etc. Focus on cleaning up not only the space but also the mental space the items take in your life.
Do it in this order:
clothes (you may split them into subcategories like jackets, t-shirts if you want)
things of emotional value and memorabilia
Keeping this order also makes decluttering easier, allows to gain the momentum and teaches how to make the decisions for later.