This question gets asked thousands of times across thousands of platforms – “How did you stiffen them?” And it gets quite a few different answers! I’m going to list links to a few of the ways.
IraRott gives you 3 ways – glue, gelatin or flour paste
http://blog.irarott.com/blog/2015/07/06/stiffening-techniques-for-crochet-decorations/
Vintage Crafts uses glue, starch, sugar and epsom salts (and throws in a link to a vintage pattern as well! (and I got stuck browsing at all the cool stuff there, so beware!)
http://vintagecraftsandmore.com/tag/how-to-stiffen-crochet-snowflake/
They had a discussion at Chrochetville, with lots of responses!
https://forum.crochetville.com/topic/106848-whats-your-favorite-way-to-stiffen-thread-snowflakes/
Hope this gives you some ideas!
Christmas Year Round 