C.J.
I did this a few years ago - but didn't have highlights, my whole head was blonde and about shoulder length. took two colorings. I colored with one bottle and let it "settle" for a day or two, then used a softer brown the second time and it "took" I really liked it, but I also was in Grad school and didn't have to show up at work while it was "mellowing"
I would choose a lighter shade than you think, concentrate on putting it on the highlight area first. Let it sit there about 10 min longer, put it on the whole head for recommended time and then follow wash instructions, but be ready to possibly still see highlights and re-do in about a week.
Good luck - going dark to light is much easier than tring to pull color to go light.