Can somebody please give me directions on how to add toner to blonde hair to take out yellow tones?

I already have the toner but how i know how long to leave it in so it tones just right and not gray or purple? I am only doing the roots and want to have even tone?Can somebody please give me directions on how to add toner to blonde hair to take out yellow tones?
Every toner varies. I'd say maybe 10-20 min? You need to read the box or something. Toner that comes over the counter does not work as well as a professional, It might spot on your hair.Can somebody please give me directions on how to add toner to blonde hair to take out yellow tones?
ABOUT TEN MINUTES. A TONER SHOULD NOT BE LEFT IN LONGER BC IT WILL START TO LIGHTEN THE REST OF THE HAIR. AND THE ROOTS WILL BE LIGHTER BC OF THE HEAT FROM THE SCALP. NO MORE THEN 10 MINS!! WITH 10 VOL ON DRY HAIR AND 20 VOL ON WET BC THE WATER WILL BRING THE VOL DOWN