While I wouldn't say the Ken Combo is something super advanced or a "leave it to the pros" kind of thing, I do agree that you should focus on getting your movement and control of Marth down before you get into doing specific combos.
In the combo, the double jump is used and immediately followed by the Dair. The jump helps stop the downward momentum you'd get from doing the Dair by itself.
If you feel like practicing the combo specifically, you can mess around with shorthop Fairs on CPUs on Final Destination. Note that tippered Fairs send the opponent up, and regular Fairs send them away. You'll practice keeping your opponent in hitstun by attacking them at the right distance. Once they're pushed offstage, you will be setup for the spike. But you have to adjust your timing and spacing depends on their %, their DI, and the character being used (CPU opponents can make it seem easier than is). It's for this reason you should work on your general movement around the stage, on the ground, in the air, and around the ledge so that you can better understand what you need to do.
Since you're mastering the advanced techniques, check out this video:
https://www.youtube.com/watch?v=_IACW5mltqY
(Marth Trials)