A little fuzzy on the steps and terms for getting silver/grey hair. Help please!

So obviously I want silver/grey hair. I have both 10 and 20 vol developer, bleach, and wella t18 and 050 hair color. I have bleached my hair before so that part I'm not too confused on.

Just bleach powder, developer, wait and wash out.

I do have a couple questions on what comes after this, and what the difference between terms are.

What is toner and how does it work? The T18 says it's a toner. So do I mix it with developer? I've never used a toner before.

The 050 says it's an "additive". What does that mean and how do I use it?

Currently, from my understanding the process would go like this: bleach hair, then mix 2:1 ratio of T18 and 050 (as suggested by the women at Sally's) with developer (??? Do I mix it with a developer? I must mix it with something or there's no way there'd be enough to cover my hair). Then let that sit and soak into my hair for appropriate amount of time. Wash out and hope I didn't fuck it up.

Also, would it damage my hair a lot if I did this all in one day? I know my hair would be able to take the bleach pretty okay, but would it be a bad idea to bleach AND dye it in one day? Thanks for any help!! Before this mostly I dyed my hair from boxes at CVS, so this is kind of new for me.