M.L.
I'm a stylist. It's best to use color on clean, dry hair. I'm giving this direction assuming that the gray is mostly colored on your hair and you are mostly needeing the roots covered. Apply a bit of vaseline to your skin by the hairline and ears to prevent staining, careful not to get it in your hair.
Apply color to the hairline first, then apply to the regrowth (roots) of the hair until the timing is done. I can't remember what it is with Wella, about 30 minutes I'm guessing. Then, lightly spritz the rest of your hair with water (just a tad bit damp), add a small amount of conditioner to the color you have left in the bowl, and apply it to the rest of your hair and comb it all through very well with a wide comb. Wait about 10-15 more minutes and then rinse it out until the water runs clear. Then shampoo and condition your hair.
If you are needing to apply it over your whole head and not just the roots, than apply roots to about 1 inch from the ends, again on very lightly damp hair. After about 30 minutes, comb the color down to the ends, wait about 10-15 more minutes than rinse/wash/condition it out.