When applying all over color, there is no right or wrong way to color your hair. To ensure an even saturation, I recommend following the sectioning mentioned earlier, applying color to the roots of each section first, and then going back through and applying the rest of the color to the middle and ends.
Alternatively, if you are doing this process alone, you can also part your hair vertically from the crown of your head to the nape to reach all of your hair.
This is always a messy process, but for the least amount of frustration you'll want a non-metallic bowl, an applicator brush, clips and gloves. I've also found that a 50/50 mixture of bleach and water removes most hair-dyes from bathroom
surfaces ;)
Happy Painting!