Hi all,
First of all, welcome to Flightgear and the forum!
When you first launch Flightgear, go to the Locations tab. After you go there and select the appropriate runway you want to take off at, there is an option below saying, "start in mid-air" and you can set the plane on an approach to the given runway with the speed and altitude. However, the main problem to this is that it starts you off mid-air with a fully cold and dark aircraft. After I start Flightgear with these settings, the plane instantly falls to the ground. If there is a way to get around this, I don't know about it yet, but someone on this thread might.
The way I do it, is that I launch Flightgear on a runway and run the whole traffic pattern and perform a successful landing. Then I go to Replay mode (View/Instant Replay) and it will give me a replay of my short flight. In the replay bar, there is an option saying, "My Controls!". When on final, click that button, and it will give you full control of the aircraft. This allows you to practice your landings however many times you want!
Happy flying,
Delta5142