blonde turn 2drk brwn w/red tips

i hav white an blk mixed texture hair. it is short in length. at the moment it is lit brwn an blonde. i want to change it to drk brown with red tips. how do i go about doing htis . should i color ALL my hai drk 1st or just the roots drk brwn so the tips wont have much to lift. I have taking cosmo clas in the futurean do know a lil about hair.