"how to treat a bruised metatarsal"?

Get weight off foot. Contusion(bruise) metatarsal requires weight to be removed from forefoot. This can be accomplished in a variety of ways. Padding the forefoot with relief beneath the effected metatarsal is the easiest.
Several things. The best way is to remove pressure from the area. I would also use ice and ibuprofen. If no relief see a podiatrist to rule out fracture or neuroma.
Icing is good. Does it hurt when you walk? If so, them immobilization maybe required with either a surgical shoe or cam walker boot.
TREATMENT. I guess a "bruised" metatarsal is a sore, but not broken, metatarsal. Technically, a bone bruise can only be detected on MRI, not x-ray. Regardless, rest is probably best. I would use a wooden-soled surgical shoe, allowing you to walk on it.