In every code review, make sure to say at least one nice thing about the person's work. People tend to provide almost exclusively criticism, because praise feels awkward to deliver. But it becomes easy with practice!
Sometimes you can't identify a piece of code that feels worthy of praise. If this happens, compliment the approach, or the effort, or the problem being solved.
"So glad somebody is taking care of this!"
"Thanks for remembering to write tests!"
Make sure to read your compliment using different tones, to check for sarcasm. The above can sound sarcastic. Here's my second attempt:
"Thanks for adding tests for this! 😀🥇"
The exclamation mark is your friend!
The exclamation mark is your friend!!!!!!!!!!!!!!!!
"Great description on this PR! Saved me a bunch of time! 🏆⏱"