A lot of dealers in NoVa can do the paperwork for you, since this happens all the time.
If not, get a bill of sale and make sure they do not charge tax (or if they do, it's MD tax). Then, you can make an appointment with MD MVA. Bring the bill of sale and your credit card, as you'll need to pay MD tax at that time.
Oh, and the "dealer processing fee" is higher in VA. If the dealer has an MD location also, threaten to buy from there due to lower dealer processing fee. They'll just drop the price accordingly.