Pushing a stray sk_buff to the NIC

I'm trying to take "hand-built" sk_buffs with little more than some data
and a dev member and push them to the NIC for transmission.  I would
like to simply give them to dev_queue_xmit.  Does anybody know what
state I should have them in before handing them to dev_queue_xmit? 
Should skb->data point to the start of a MAC header or an IP header?

Also, given an IP address in skb->nh.iph->daddr, what's the easiest way
to get the appropriate MAC address?

J