Making aircraft work better in Kerbal Space Program (KSP)

Ah, Kerbal Space Program. Such a mystery at times.
Aircraft can be frustrating and many players give up on them and concentrate on building bigger rockets instead. You don't have to though!
Aircraft are fun, and space planes are way more cost efficient than rockets. With a little help, you can unlock the full capability of your designs.

Have you ever felt like asking questions like:
Why do some of my planes fly fast and others struggle?
Why is my plane so unstable?
How come it won't roll straight?
How come it works fine for a minute and then flies increasingly bad?
How do I make a plane that actually works?

Well if you can get past my long winded story telling, there are some answers in this video for you.

If you want to skip around:
Wing tuning 7:06
Fuel balance problems 19:07
Runway problems 22:40
The woe of undersized wings: 24:09