I want to put an under-colour in my hair but I'm not sure how to go about it...?

Ok, I have layered dark brown/black hair with a few blonde bits underneath from when I had coon-tails. I'm wanting to put an electric blue under-colour in my hair (it's in fairly good condition as the last time I dyed it was months ago) but I'm not entirely sure how to seperate and section my hair to dye it to get the best effect. I would go to the salon but the last time I had my hair dyed 'professionally' they screwed it up and charged me �100 which put me off getting my hair done there. Could somebody please help me out? I literally just want advice on sectioning my hair, not on where to go to get it done. I already have the dye and the lightener to get it all done. Thankies :)