Generally speaking, with Corrin you don't want to focus on combos. You should focus on covering landings, ledge options, and keeping your opponent in the air for juggling. In other words, just keep them in a bad situation and rack up damage that way. If you happen to get a combo, great, but don't over extend, otherwise Corrin could end up in a bad situation. If you want to improve your combo game though, you need to identify your opponents habits, and know exactly what Corrin is about as a character.
If you wish to improve your combo game with Corrin, try to utilize d-tilt in neutral more, and go for more tomahawks, since it'll condition your opponent to not shield as often. Also, you can go for a footstool with Corrin at certain percentages after you hit with a second fair, fast-fall with a Nair(be sure to follow the direction they fall), generally the percentage it works at best is from 10-50 depending on the character of course. I find hitting them with Nair first works better. It can rack up a ton of damage, but don't go for that to often as your opponent can catch on and it'll stop working at higher percentages.
Just remember though, Corrin isn't a combo character, and generally speaking, combos can only take a character and player so far if you don't know how to properly utilize the character.