The art of writing a greeting card is one we're all too familiar with given how many cards we send on the daily. But the main piece of advice is to follow your heart (or gut -- whichever fails to lead you astray). Basically, avoid overthinking this. It's a miss you card and all you've got to say is that you miss them... Everything else is icing.
Some suggestions on what to write in your miss you card:
- Reminisce on a shared memory (but make it brief since you've got limited space).
- Tell them you're thinking about them.
- Look forward optimistically and consider suggesting plans for a future meetup or even a FaceTime call.
Those are just a few things you could add into your message, but remember to keep it sincere, personal, and brief.