I just did it for the first time this week too. I read and read and read online and everyone said different things....different amount of vinegar..different times to use it (before or after condish) so i ended up doing my own thing...i'm not saying how or what u should do..but this is what i did and it turned out great.
First, i distilled some water..2 cups of water..and i added 1/4 cup of apple cider vinegar and let it cool.
I had shampoo'ed my hair the day before with a non-sulfate shampoo..so i only co-washed..i put the conditioner in my hair and rubbed and massaged and then detangled my hair. I rinsed out the conditioner..and then slowly poured the vinegar mix on my hair to make sure it got completely coated (make sure u dont get any in your eyes..it burns like a b*tch)...gave it a couple of minutes to settled. I noticed the difference immediately. my hair was nice and smooth and shiny. I then added my leave in conditioner, oil, and styling product.
I really really liked how it came out. The only thing i would do different next time is add less vinegar because it came out a tad bid harder than I would like but other than that..i completely noticed the smoothness and shinyness in my hair. Loved it. I will definitely be adding this to my routine once or twice or month.