Thoroughly rub shampoo into your scalp around your roots. The amount of shampoo you use should be about the size of the inside of your palm, depending on length. (The amount also varies depending on hair types.)
Don't be too rough; if it hurts, try massaging it instead with your head tilted back under the shower until theres some white foam in your hair. Leave it in for around 5-ish minutes.

Afterwards, tilt your head back again & place your hands on the beginning of your hairline with your hair backwards (like in the image)

and gently press down, doing a motion away from your forehead until theres little to no soap left. Afterwards, get a bit of conditioner & apply it to the tips of your hair, up about maybe 1-2 inches. Leave it there for however long you see fit, for me I tend to do it about 5 minutes. Afterwards, wash it out and you should be fine.

If your hair is thicker like mine, you'll want to repeat it twice to make sure its thoroughly cleaned. If you clean your hair less often as well (whether because of depression or anything), you'll want to do that.
If for whatever reason you can't tilt your head back (mobility issues, strain, etc) then you can lightly tilt your head forward and just put your hair ontop of your head in a strange sort of bun. It won't wash nearly as well and you'll have some knots, you'll also get some soap in your eyes but if its either that or not washing your hair at all, it's better than nothing.

*please feel free to share below whats worked for you, I am purely speaking from personal experience; my hair isnt that long, it's just a bit further down than shoulder level, and not everyone has thick hair.