Lots of bleach :( And toner, usually bleached hair has a yellowy tone to it, especially if you're lightening it from a darker color- make sure to condition and treat your hair as needed, since bleach can be really dangerous! Once you have a light enough shade of blonde, buy a toner, usually a level 9 or above (hair color works in numbers, 1 being the darkest and 10 being the lightest) and MAKE SURE it has a letter in it like v or b- these mean the color has a violet or blue base, which will neutralize any brassiness.