How do you make a shampoo cap (amounts of peroxide and shampoo) how long on hair, what does it do?

I use a shampoo 'cocktail' (half oz. 20 volume peroxide and enough shampoo to lather the hair) on my very light brown hair. It is shampooed into the hair and left on about 5- 7 minutes. It doesn't really show up the first time, it takes a couple of treatments to notice the blonde highlights. Once the highlights are how I want them (2 or 3 treatments), I repeat once or twice a month, any more than that and I get roots that need to be covered up.How do you make a shampoo cap (amounts of peroxide and shampoo) how long on hair, what does it do?
We used to mix peroxide and just enough shampoo to suds up - to a new color if it seemed to dark on the ends. And left it on anywhere from 5 to 10 minutes. Measurement depended on amount of hair. Never used left over color or water. Water will just dilute the peroxide.How do you make a shampoo cap (amounts of peroxide and shampoo) how long on hair, what does it do?
That is an old term! Most likely it is still made with the bit of colour left in the bottle and a couple of drops of shampoo mixed with some water, shaken together, and left on for no more than about 5 minutes at the end of a colour session. The reason for this is to refresh colour that has faded throughout the hair strand. It is not meant for serious colour work. I was a hairdresser for many years.