This video will answer my most asked question ever!! My hair colour is light ash brown and I use the Loreal Excellence Creme (link below). For £6, I can retouch my roots or dye my entire hair myself, no more overpriced hairdresser visits. I use 1 bottle to dye my roots and 2 bottles if I was to dye my entire head. If you were to dye your entire head, I would just apply it all over, if you have roots to begin with then dye your roots first, let it work it's magic for 10 minutes then apply the rest of the mixture to the lengths of your hair.
Disclaimer: Everyones hair type is different, it may come out a different colour on each and every one of us. This is just how I dye my hair so I hope it does help.
My hair type is oily. Its also best to dye your hair when it's dirty, this will protect your hair due to it's natural oils, if you dye your hair when it's fresh and clean, it will damage your hair more.