Well, since you're not willing to see a professional (I understand why), just try your best whenever you're putting the dark on to not get the highlights. You could also try to pull the highlights out, stick them in some foil (Reynolds Wrap will be fine if you don't have hair foil) and put some conditioner on them. It will help keep the color off of the highlights. At Sally's, they also sell these small wands that have a small brush on the end of them for like $1 - $2 and I think it would help in applying the color to a small spot.
Good luck, if you need anymore help or anything just PM me.
