It is very easy. You get off of the train, walk and go down an escalator. There are a few stairs but then you are in the Union Station Metro. The walk is about 5 minutes or so, perhaps a total of one city block.

You will be transferring at L'Enfant Plaza and taking the Yellow line of course. The transfer is easy -- no stairs at all. Just an escalator. Not all that much walking.

Check the Metro schedule as Sunday trains don't run all that often. This Ride Planner can help you.

You can generally use a credit card at one of the machines where you buy your tickets in the station. Usually it works. I would bring some cash in case it isn't working. The machines take the usual bills and change.

Lots of people take luggage with them on the Metro. Good luck!