This achievement is really self explanatory and shouldn't be hard at all if you are familiar with Overwatch's maps and characters. The flag is instantly picked up when getting to it at its original base but takes a few seconds to be unlocked again if it was dropped by a fellow teammate. I recommend working with your team to be to at least clear most if not all of the enemy team and get them away from their flag and return to your base quickly. I also recommend using a quick character so you can get in and get out of the area as quick as possible. These include but not limited to Tracer (who I recommend the most), Genji, Lucio, Soldier 76 and Sombra, but you can use anyone if you play smart, like using Winston's jump correctly.
EDIT: If you use movement based abilities such as Tracer's warp or Soldier's sprint, it will cause you to drop the flag (Thanks to SergeantMajorAj for the tip). Tanks can also work well as they can take a lot of damage like what HUN playmore stated in the comments. If anyone has anymore useful tips, post them in the comments and I'll be glad to update this.
Some notes:
- You can only get this achievement during the Chinese New Year events, which happens usually every year from late January to mid February.
EDIT: Capture the Flag is now available outside the CNY events.
- This can be unlocked in either the normal CTF events or the competitive one.