How to cover stubborn greys with 20 volume developer?

How do I pretreat stubborn grey hair with 20 volume developer? I do at home coloring with department store box permanent color. The girl that cuts my hair said to saturate my hair with the 20 volume developer alone for about 30 minutes, wash it out, dry it and then color as normal with the color and the developer in the box. This seems like a long process. Can I put the 20 volume developer on my roots, wait 30 minutes and then mix the box color with the box developer and put that on next on top? Do I have to wash the 20 volume developer out first?