You don't need to buy developer with a box dye kit, it's already included. All box dyes contain instrucions, hair color, developer gloves and/or processing cap, sometimes a deep condioner as well. Box dye kits are intended to enhance your natural color/tone, cover grey, or or go only 1-2 shades lighter. Since your hair is dark brown and you're using a blonde hair dye, it's only going to come out an orange red shade....if you were actually trying to go honey blonde you will need to lighten before coloring. I tried Dark and lovely in Honey Blonde once a long time ago (not sure why because I'm not a big fan of most box dye kits) and it dried my hair out terribly even after deep conditioning. As for the clip-ins, I suggest waiting til after you color to get a shade to match. It's easier to dye your natural to match extensions rather than the other way around.