What have you tried so far ?
If you asked 20 people you'll likely get 15 different answers .
You could buy 5 cheap products that don;'t work
Personally , mine are -
Superdrug Argan pre-treatment ( orange tub with a green label) or PK Elasticiser ..
Loads of people don't rate the PK , you do have to use a few times to get effect.
Coconut oil brings my scalp up in lumps even if I'm careful not to get it on my skin
Olaplex for me (3+6 ) I don't dye my hair - was meh
I used colourless henna , made my hair so dry it tokk ages to recover .
Whatever you use though , damp hair , apply , clingfilm and a towel/hair turban . Let the heat from your head do the work.
Rinse out .
You could try conditioner only to wash with , leave out the shampoo
Cut off a few centimetres, once your hair has gone frazzled there's no return really .