Draw a template of desired shape onto paper and attach to the mason jar with double-sided tape. Apply paint to exposed areas. Once the first coat of paint has dried, carefully remove tape and apply next color of paint.
Message in a bottle guest book (people can write messages of their well wishes, or a piece of advice, or answer a question you ask them. For example, 'what is your favorite memory of the bride and groom' or 'what should we name our first child', etc.