How to blend Dark roots to platinum Lengths.

Hi!
I'm so happy I found this forum. I am desperate to update my roots since it will still be a long while til I get to see my stylist again.
Just a quick background: I went from dark brown(a bit of grey in there too) to platinum almost a year ago. My stylist did it very gently over 3 months. I usually visit the salon every 3-4 months and she'll blend my roots into the platinum so that you can still see my natural color near my roots. I prefer this method because I don't get the "stripe" growout. I'm sure theres a more technical way to explain this.

I've attached current pic. My platinum has a rose color mask that will fade away soon. I will probably wait until its gone before I address the roots.

What I "think" i need to do is grab a 20 Vol developer, a lightener, and a "beige blonde" or "ash" toner, and some foils as well. I think this youtube video shows me how to apply the bleach to get the desired effect. But I am so lost its been a really long time since I did my hair myself, but I'm desperate. Thank you!