box dying my whole head

2 months ago, i attempted to dye my hair into one even color when my roots had grown in but, it ended up being back to black basically instead of brown. to fix this, i tried lightening my hair color with vitamin c pills and clarifying shampoo which did work however my roots are still a little darker than the rest of my hair.

now i want to dye my entire head a lighter shade of brown with the Hello Cream dye in Warm Light Brown.

i don’t want it to end up being patchy or for my roots to be darker than my ends so how should i approach dying my hair again? thanks!