This is the first time your written word about someone becomes part of their record. Treat it that way, specific, fair, and something you'd stand behind if they read it twice.
A review written under deadline pressure, from memory, reads that way. The managers who write reviews people trust are the ones who weren't reconstructing the last six months from scratch the night before.
"Good work this quarter" tells someone nothing about what to keep doing. "Your rollout plan for the billing migration is why that launch had zero incidents, keep building that kind of plan before every risky change" tells them exactly what to repeat. Specific enough that they'd recognize the moment you're describing.
The Performance Review Script covers the structure of the conversation itself.