The best way to do focus fire

For those in clans skilled enough to do it regularly, do you call out the target each time? Do you have a list of priorities? Do you have one soldier be the primary, and everyone tries to hit his target, or does it just come naturally?

Also, how do you practice this?

we just say "so and so on the right/left/by the thing" and proceed to kill them

medics r almost always the highest priority targets

we dont really practice... we scrim here and there and have matches once or twice a week

The easiest way to focus fire is to stay together in a blob as a team. That way it's possible to see what other people are firing at and assist where possible. Unfortunately it's not always easy to stay together, and some maps require attacks where you split your team into smaller groups. (e.g. Gravel Pit)

One nice tactic is to have the two soldiers both use shotgun against the same scout. Focus fire has become a lot more important since crits were disabled in 6v6. It's still pretty difficult to execute since there are so many obstacles in this game that prevent multiple people from having line of sight on the same player.