Most people spend more time choosing the flowers than writing the card. Then they get to the message box and write something like "thinking of you!" and immediately feel vague about it.
That's not a 'you problem'. It's just that blank cards are strange. You have about three sentences to say something that doesn't sound corporate, doesn't sound stiff, and actually lands.
Here's what actually works.
Write to the moment, not the occasion
"Happy Birthday" is fine. "Happy Birthday, I can't believe you're 30 and somehow still cooler than me" is better. The difference is specificity. Greeting cards are generic by design. Your card doesn't have to be.
Think about what's actually happening in this person's life right now. Are they going through something? Did something good just happen? Is this a particularly big or small birthday? Write to that.
Start with why you sent them
"I sent these because" is a genuinely useful sentence opener. It forces you to get specific. "I sent these because you've had a brutal few weeks and you deserve something pretty on your bench" is a real message. It tells the person they've been seen.
You don't have to be eloquent. You just have to be honest.
Some prompts that help
If you're still drawing a blank, try finishing one of these:
"I was thinking about you today because..."
"These reminded me of you because..."
"You deserve flowers today because..."
"I wanted you to know that..."
None of them sound like a template once you fill them in with something true.
Short is fine. Short is often better.
A card doesn't need to be long to hit hard. "You're one of my favourite people. Enjoy these." is a complete card. "Thank you for being exactly who you are" is a complete card. The goal isn't volume. It's a sentence or two that feels like you wrote it, not copied it.
What to avoid
Skip anything that sounds like it could be a bumper sticker: "Bloom where you're planted," "You're a ray of sunshine," that kind of thing. Not because they're wrong, but because they don't say anything about the specific person you're sending flowers to.
Also skip apologising for the card. Don't write "I'm not great with words but..." The card you write after that sentence is almost always better than you think.
One last thing
If you're ever completely lost, just describe the flowers. "I picked these because they're a bit wild and so are you" is a card. "These are bright and loud and they made me think of you immediately" is a card. The flowers are already doing the work. You're just adding the reason they're from you specifically.
That's the whole job.