To rate the games, click the game name from the list of submissions here.
https://itch.io/jam/next-adventure-jam/unrated
Or in the top right of the game page there is a small button which says "Rate for The Next Adventure Jam" which will take you to the rating screen.
As far as voting goes, the voting should be based on your criteria about what makes a game good. It's entirely individualistic. If you love a game, give it five stars, if you really dislike a game, give it one star. Also adding comments with constructive criticism is good, as I'm sure you've discovered for your own game. Please don't say bad things that are not constructive though. The purpose is to support one another, and to help authors learn something too, via feedback.
Chris