When going from brunette to blonde, it is very important to analyze the condition of the hair. Determine if the hair is in a fragile state or healthy. This all determines the correct process and steps to take.
Depending on if the hair is artificially colored or not will also determine the appropriate process.
Use a very gentle bleach on the hair if previously colored. I recommend Schwarzkopf Professional Blonde Me. Blonde Me can lift up to 9 levels, and does so all the while taking care of the hair.
Apply Blonde Me lightener with the desired processing solution from mid lengths to ends. Leaving the roots for the second process. Once the mid lengths have recessed for roughly 15-20 minutes then apply to the root area. Natural heat from the scalp will cause the roots to process faster. If ends are very damaged, apply lightener to them after you apply to the roots; they will take less time to process.
Either apply all over or use foil to separate each section. Do not fold your hair into foil packets as this will cause separate heat zones, causing the hair to lift unevenly.
Once lightened to the desired level, tone the hair to the appropriate shade. Always reference your color wheel and apply the opposite color of what the hair is, to achieve the desired tone. Finish with a moisture-rich deep conditioner for healthy shiny hair.