How to access luggage/ recheck bag?

You don't have to say anything to anyone. If Washington is your first airport in the US, you will clear immigration and customs there. First, you go through passport control. Then you go through a baggage claim area to get your checked bags. You have to carry the checked bags past the customs agent and hand in your customs declaration. They may or may not send you to secondary inspection where they scan your bag and/or open it for visual inspection. After you go through customs, you drop off your bag to be forwarded to your next flight. While you have the checked bag, you can transfer things between carry on and the checked bag. However, you need to be aware that you will have to go through security again before you can get to your connecting gate. If you bought any liquids after security in Germany, you'll want to put them in your checked bag or they will be thrown out at security. Similarly, you don't want to transfer anything from your checked bag to your carry on that is not allowed in a carry on.

Normally, you would be unable to do that. You can't access luggage between flights, if it's been checked through.

However, in this case you HAVE to get your bags in DC so that you can go through customs. So you will be able to access it there. There is nothing you need to do or say - you have to clear immigration and customs in the point of entry into your destination country, which is DC in this case.