These may sound a bit generic, but:
-Use your binoculars to mark as many enemies as possible.
-Take your time and observe guard patterns before moving.
-Don't sprint unless you know exactly where you're going.
-In general, stay crouched when navigating unfamiliar territory.
-As usual, tranq headshots are your best friend.
-Try to hold up guards if you can get behind them, and don't leave them lying around in the open.
-Don't underestimate the guards' field of vision. The AI in V is near-realistic in that regard, so assume that if you can see them, they can probably see you.