Protips:
1. Start Small, if it gets to big it will be very hard and boring to finish.
2. Try to make it interesting, not just a big fire fight. Add a good goal.
3. DO NOT SPAM. Spamming your walls and forts is just plain bad.
4. Don't make it too small or inclosed, it will just be a big blood bath.
5. Add plenty of cover and props, to make the battlefield look and play well.
6. Make lots of secert tunnels and side passages, make sure there are multiple ways to your goal.
7. Balance the teams build and weapon wise. It's no fun to play a TDM were one team is really overpowered.
8. Finally, if you get bored building, take a break. It will help you get good ideas and make the process more fun.