Rather than learning combos know when to use what. This is where the Marth Video compendium comes in (or FE Lord's video corner), watch when and why they use f-air, n-air, u-tilt, etc.
Some things to ask yourself: Why did he choose Nair over Fair?, how high was the opponent's health?, how close were they from the edge?, What was done immediately after the attack, How did he approach?, What was the opponent doing?, Where was the opponent?
...and stuff like that...
Other than that, you need be able to SHFFLC your F-airs really good. Also you should know how to follow up a F-air with something.
Basically, you need to be more spontaneous I suppose...