The first stop for your last mission is the Presidential Metro. You can access it by going to the entrance in the D.C. Ruins. Go over to the marked spot by the East Capitol Building. Go into the utility route and go down a new ladder into the Presidential Metro. If you go to the White House Crater, then you missed the entrance.
The Presidential Metro itself is pretty straightforward. You just keep walking down the tracks. There are some turrets and sentry bots that you have to watch out for during your trip. You should be good at killing them by now. Just charge forward and target their arms in VATS. If you manage to cripple them, then you basically get 5 free seconds to finish them off without taking any damage. Clear a path through the defenders. Your goal is to follow the line until you get into the next office with M.A.R.Go.T. If you don't have much skill in Speech or Science, then you might want to look on the train cars for the Senate ID. You'll find the ID next to a skeleton on a train car right after you cross over from the first corridor to the next. That lets you skip the test. It shouldn't be a problem for any moderately skilled character though. Just save and try again if you fail.
Once you pass the test, M.A.R.Go.T. asks for your help in securing the Presidential Metro. Apparently a whole bunch of ghouls are there. Arm yourself and prepare for one heck of a fight. If you are doing this at a high level, then be ready for a tough fight if you plan to help M.A.R.Go.T. The sentry bots just can't handle the feral ghoul reavers that show up on high levels. You can barely handle them, thanks to the tight space. The only real hope is that they will pointlessly attack the turrets and expose themselves to a lot of attacks.
You don't have to clear the Metro. If you want, then you can just put on the ghoul mask and go forward to the end of the tracks. There will be one sentry bot by a fuse box and the train car. You can just kill him to get the fuses. It's a simple repair task to pop the main fuse in and start the car. If you kill the ghouls, then M.A.R.Go.T will thank you and the sentry bot will take care of the repair for you.
Get into the train car and start your journey. You'll end up at Adams Air Force Base.
It's time to quickly get your new toy before the Enclave figure out that you're here. Kill the few soldiers and start walking toward the marker. Go around the back of the nearby control building and look for the large box. Pick up the Tesla Cannon and the ammo here. Grab the orders to learn about Tristan's plan and trigger the Brotherhood of Steel assault on Adams Air Force Base. You are taking the brunt of the punishment though.