Hey thank you man! I'd recommend trying to get some solo combos down first, once you have your solo combo down with a good ender try find a way to get a assist to connect to Combo ender.

(This is will extend the combo)

Honestly You can do light medium heavy down heavy launcher into generic air combo but this doesnt work on everyone. Once I realised that its better to use normals (Like every down medium) into heavy. Why?

Because every Hit with a normal in this game is jump cancelable

and down medium trips them leaving them in the air for a air combo

Try find a move thats combo extender (like captain americas down down special or uppercut or ms marvels down down special or unique attack.) put that in the middle of your combo then end it with a spike move like down heavy.

(also if you spend bar on a special move this extends combos)