It's hard to offer "in-depth" help when you are asking for help in general, against all characters.
The first thing you need to understand is that Jigglypuff has amazing aerial mobility. You should always be able to control your position in the air to your advantage, and you should always be in the air. Don't stay on the ground ever, unless you have a very good reason.
Second, all of your air attacks are really good. Like, really good. Each one has a specific purpose:
-Fair is your most important, because it kills if you hit with the beginning of it. Because of stale moves, you need to save your fair until you can kill. If you hit with the second weak part of it, normally you can automatically chain into another fair, this time a strong hit.
-Bair is next most important. You *want* to use this move as much as you can because you *want* it to be stale. When bair is stale, it can combo and carry opponents across the stage or even to their death. You need to practice through the air facing backwards. This is absolutely critical.
-Then comes nair. Nair comes out faster and let's you get out of sticky situations. Nair also stays out longer, meaning it's a safe move to jump out into the enemy with.
-Dair is your fastest aerial. It can do excellent damage and on the ground it can link to rest. Dair's multi-hit nature means it is a very good option for some approach situations, but a very bad option for trading hits in the air.
-Uair is able to juggle opponents for great damage; practice on ROB at various percents to get a feel for it. Also, see if you can predict an air dodge and rest a mid-air opponent as they come out of the air dodge for an easy KO.
Finally, you have your wild card: Pound. Pound should always be used in the air (never on the ground), and in most situations it should be angled upwards as you do it. Pound has fantastic priority, but plenty of ending lag. Pound can be done off stage to control your position as well as help you recover. Pound is great against shielded opponents, as long as you do it so they can't grab you or hit you with a U-Smash or u-tilt. If the enemy is attacking aggressively, try Pounding downward; it is rare that anyone has a stronger diagonal priority option than falling Pound.
One last thing: master shield grabbing. Whenever you touch the stage, opponents will try to rush in and take advantage. Jigglypuff's throws all do over 10%, so you stand to gain a substantial amount by countering them in this way.
That's it. Learn the purposes of her 6 aerial moves (includes Pound) and get a feel for them, and you will begin to master Jigglypuff's aerial game. Don't worry about Rollout or Rest too much, you shouldn't build your game around gimmicks to begin with... that's only lead to bad habits. Those moves will become useful in many situations, but only AFTER you have got the basics down.
Sound good?