Ok. So because of some weirdness from the construction and another line being out of service, There are two different blue lines right now. It won’t make a huge difference for you, but the digital signage on the side of the train may say either of the two destinations. Don’t let it confuse you :)

When you leave DCA to go to IAD, follow the mounted permanent signs for the blue line toward “Largo.” I think some signs may actually say something like “trains toward DC” or whatever. The digital signage on the actual train car, because of the construction, will say either Largo or New Carrollton - either is fine!! You will ride five stops and arrive at Rosslyn station, be sure to step off the train here. Go down to the lower level platform (don’t exit the station) and board the silver line train to Ashburn. After many stops you will arrive at IAD. (When you’re boarding the metro from DCA, you DON’T want the trains that say either Huntington or Franconia-Springfield. Wrong platform/trains!)

When you land back at IAD and need to reverse the trip, it’s easy. The only line at IAD is silver, just be sure to board toward Largo (not Ashburn). Once you exit the train at Rosslyn, head down to that same lower level platform you’ve seen before and switch to a blue line train toward Huntington or Franconia-Springfield - either is fine! And five stops later you will be at DCA.

If you load 20 bucks when you get your card out of the vending machine at DCA, you should be fine for all of this.