A question on hair bleaching..please help?

Yesterday I bleached my entire head with bleach twice. I was dark brown and wanted to go platinum blonde. However I achieved the the colour I wanted but only at the top half of my hair lengths. I did some reading and I didnt realise that u are supposed to do the roots last cause they develop quicker. SO I now have to bleach just on the midlenghs one more time to get it more even? How do I do that without mucking it up. I have not put a toner in yet so its still a lil orange. Thanks