This is what you do to shoot the practice planes. It works in all practice missions:
Don't turn to try to get behind the enemy planes, You can't outturn them with a Sopwith or a Fokker triplane.
What you should do if you fail to shoot the plane down on the first encounter is: not turn , but just continue flying straight past as if it were a car in the opposite lane of a highway. The enemy plane won't turn and go after you unless you turn first.
Then look at the symbols on the map. The white triangle is you and the little red dot is the enemy.
If the enemy sits on your tail, you can't shoot him. If you have let him pass you without turning, just wait, and you'll see the distance between you increase.
When it increases no more, turn while looking at the map, until the open end of the white triangle points right at the moving red dot which is the enemy. Then fly towards him and shoot.
After having downed the plane, get close to the airport by flying low, following the shore of the lake, so that the tanks can't see you.
The tanks are very accurate and will shoot you down at a long distance, but their turrets turn slowly and they need time to aim, so if you avoid flying in straight lines or straight towards them they will have a hard time hitting you.
Bombing the hangars in the practice missions is almost useless as practice for "real" bombing, because your plane acts very differently when you're approaching, turning, diving, and dropping bombs from what it does in the multiplayer game.
In the blizzard mission (mission 5, "dogfight") the flickering motion of the snowflakes on the screen makes it difficult to see the enemy planes. In the beginning the snow is just beautiful, but soon it becomes vexing and in the end it's outright horrible. I'm sure it could trigger an epileptic fit in a susceptible person.
If it weren't for the snowflakes, mission 5 would have been good practice because the planes act almost like they do in close combat in the multiplayer game. You can still fool them into giving you aiming distance by flying straight past them, but if you use the Fokker triplane it's in fact also possible to shoot them down in a turning game.