After obtaining the COMBAT KNIFE, return to the Communications Tower Base Camp (the one near the SUPPLY SHACK) via fast travel to complete the Capture the Flag Challenge. This involves cutting down a series of old Soviet flags. The first Soviet flag (1/7) is located on the porch of the SUPPLY SHACK. Stand next to the flagpole and press Y to cut down the flag.
Then go down the wooden stairs below the shack and base camp...
...to find the next flag (2/7). Cut it down the same way.
Continue down the wooden stairs. Then slide down the snowy slope and climb on top of the rusted water tank to cut down the next Soviet flag (3/7).
- OR -
If you're coming from the SAWMILL building, the third flag is on the other side of the frozen stream just beyond the ANCIENT CISTERN CHALLENGE TOMB entrance.
There's another Soviet flag (4/7) on top of the abandoned SAWMILL. Watch out for possible enemies whenever you revisit this area. (In this screenshot, I've gone around to the south side of the building to get a better view of the flag.)
Head downhill to the Logging Camp Base Camp to find the next Soviet flag (5/7) near the fence above the river. (Of course, you can also fast travel back to this camp and find the flag that way.)
The next flag (6/7) is on the way to the COPPER MILL. Follow the road along the river and head for the wooden guard tower. The flag is just below the tower. Be on the lookout for wolves and Trinity scouts as you explore this area.
The sixth flag is a little easier to see from the other side.
The last Soviet flag (7/7) is a bit farther to the southeast, right next to the Copper Mill Yard Base Camp. When you've cut down all seven flags, you'll get an XP bonus—plus Expedition Credits if it's your first time completing this challenge.