Fiber Twig: Midnight Puzzle

where can I review the rules?
I dont know what to do when a red diamond with a number inside appears

You don't need to do anything as far as I can tell. The game is giving the red diamonds to you. You can buy lightening, the hour glass and the globe with them. Lightening lets you remove a piece you cannot use, the hour glass stops the new pieces from appearing for 1 minute which allows you to think about piece placement and the globe will show you what type of piece will appear in the spot where a new piece is being generated. Hope this helps.