This is one of the typical grandmother beauty tipps and I'm sure many people already know it. However, this is for those who don't. ^^
When you wash you hair, the hair cuticles will open slightly, making the hair rough and thus prone to tangles. Using acid rinse makes them lay smooth again. Your hair will be shiny, smooth and easy to comb. You probably won't need conditioner anymore. ;-)
If you use shampoo bars (hair soap?) this procedure is particularly important since soap is always basic and neither hair nor skin like to be covered in base, so you can neutralise it with the rinse.
Simply mix the ingredients and poor them over your hair after washing the shampoo out. You don't have to rinse it out.
Btw. it works best when it's really really cold. I put mine into the fridge before I use it. You might consider this to cold (and I can fully understand you. ^^) but even if you use warm water make sure, that it is at least two degrees colder than the water you washed your hair with.