Try mixing it up with the developer and then adding some water so its quite runny.
Then comb it through.
By the time you've combed it through all over, it will practically be time to rinse it off.
Best to do 2-3 of these than 1 really strong one that takes it too far and gives you blue hair.
Unless of course you plan on auditioning for Avatar II The Sequel!

Good luck!

Ps. Next time, if you order a brown colour from Wella, buy the same number you usually buy but with NA after it (eg. 6NA)
The NA stands for Natural Ash and means it won't have any brassy tones.
If you hair is VERY ash then you can get an 'A' eg. 6A which is stronger.