Blonde hair to platinum?

Hi everyone!

So I currently have a half head of blonde highlights with full black underneath, and I want to go from my current blonde to platinum to create a black & white look. I have been advised against using the box kit I purchased (L'Oreal Feria Extreme Platinum Power) and after doing some research am wondering if using a toner is the best option, and just wanted to ask for some advice, and ultimately if this is definitely achievable.

The last time I had my hair done it was bleached and lightened with an ash toner or similar. Will I be able to use a platinum toner straight on top or will I have to bleach it first, and if so is this a bad idea as my hair is already quite light?

Am completely lost and unsure of what to do; I don't want to do anything to damage my hair but if there was a way to achieve the colour I'm after that would be amazing!