You'll see a Share option (marked with a camera icon) in two places: right after finishing a game round, showing your score, and on the popup that appears when you unlock a new achievement. Tapping it posts a Highlight immediately — there's no separate composer or editing step, so make sure you actually want to share before tapping it.
Once shared, all of your followers get a notification letting them know you posted a new Highlight, so it's a good way to get a bit of attention on a result you're proud of.