Broadleaf plantain--how do I prepare it for a tooth abscess?

See a dentist. Or a dental surgeon. Like all abscesses, tooth abscesses are pockets of pus that require drainage. That means that the dentist will need to drill through the decayed enamel into the infected dentin and pulp of the teeth to release the infection. Doing this may save the tooth but will prevent a bone infection of the jaw or a bacterial blood infection.
Bone infection. If a dead nerve has caused an abscess it is infected into the bone. In that case it would be unlikely that applying a medicated preparation to the surface will have much effect. A food impaction or gum disease infection might respond to surface medication. In either case the underlying cause needs to be treated or relief is not a cure. It will come back or become a bigger problem.
Not strong enough. Broadleaf plantain is a very weak anti-bacterial agent and will not be effective in the treatment of a dental abscess. See a dentist to determine and treat the cause of your problem. Oral antibiotics can be of help but again will not eliminate the causative problem.