Step 4 - Start to use gentler hair products.
Take some time to search out products that won't strip your hair of natural oils with parabens / sulphates and won't coat your hair in unpleasant silicones.
Faith in Nature hair products don't contain parabens (preservatives which may cause long-term health problems) or SLS (sodium lauryl sulfate detergents which remove grease, but dry out the scalp and hair in the process). The Faith in Nature Fragrance Free Conditioner is free from silicone (silicones form a layer around hair, at first making it shiny but as this barrier builds-up over time the hair dries out).
Suma sells unscented 'ALTER/NATIVE' Clear & Simple shampoo and conditioner (also free from chemical nasties) through refill stations at zero-waste shops too. N.B. the Suma conditioner does contain (sustainable) palm oil. This shampoo is what we all use now in the household, as shampoo soap bars were leading to irritated scalps from a build-up of dandruff / soap particles. If visiting a refill station doesn't work for you, then a local-to-you zero waste store may sell you the bulk 5 litre+ containers - thanks #OpenHouseGateshead - which will last for ages. You will need a compatible pump, or a funnel, plus a sturdy plastic bottle to decant into - I found an old Ecover cream cleaner bottle to be perfect.
On my journey towards reducing pointless plastic, I came across the (pricey but truly plastic-free) Beauty Kubes, which are cubes of compressed shampoo powder, made in Cornwall, that come packaged in a cardboard box. They are SLS and paraben free too.
You crush a Beauty Kube in your palm, wet it and form a paste to rub into your hair. It performs just like 'normal' shampoo - yippee! *EDIT* *I found that prolonged (6 month+) use of Beauty Kubes led to a build up of dandruff-like flakes on my scalp, so switching between the refillable Suma shampoo and Beauty Kubes works for me.* Personally I like the 'shampoo and bodywash' variety in the orange Beauty Kubes box. You get 27 cubes for £8.50, so they would be too pricey for those who wash their hair daily, but if you are on your mission towards happier hair then you will reducing the frequency of hair washes soon anyway.
Beauty Kubes are best kept in a reusable, plastic (definitely not dangerous glass in the shower/bath area), screw-top container so they don't get damp, plus then it is easy to get them out one at-a-time mid-shower.
The zero-plastic-packaging conditioner which I use (but not in the summer, as I find my hair can get greasy with the summer heat) is an Ethique soap bar from Holland and Barrett. I use the amazing lime-smelling, green 'Guardian' one, which is labelled for 'dry' hair but it perfectly suits my 'normal' hair. It cost £12.99 for a 60g bar, which sounds expensive but it will last for ages (the website claims it is equivalent to 5 plastic bottles of product - I believe this claim).
If I use the Ethique soap bar then I don't need a leave-in conditioner. If you do need leave-in conditioner for de-tangling, then try a tiny bit of coconut oil (such as that sold by 'The Groovy Food Company') smoothed along the lengths (not near your scalp) whilst your hair is still damp.