color advice needed

What is the best thing to mix my permanent clairol 7NP medium blond gel with, besides the #20 developer I use, to spread it out towards the ends? I`ve had a nice weave and have been coloring the roots myself and am ready to color the blond highlights in preparation for another professional weave. I told the stylist I would do the base color myself. Last time I used a gentle shampoo to mix the color with in order to get it all over my shoulder length hair after leaving it on the roots for a while. Is there anything better to use? Would plain water work? Should I just mix up more of the gel/developer combination and use that alone? Thanks for advice.