When you finish a Hanjie correctly and hit the green check button, a yellow box appears to the right - "Rate this Hanjie" - with 5 yellow then blue stars. If you put your cursor over them a tiny box appears with the numbers 1 up to 10. Just click the number of stars you think the puzzle deserves and you've rated it. (NB Don't know why there's not 10 stars and to rate a Hanjie a 9 you have to "yellow" 4 and a half stars, but...)!