Most shimeji are made by downloading an existing shimeji and replacing its images (and sometimes adding custom actions). Also, shimeji are generally made using Shimeji-ee, for Shimeji-ee, which you can find here: https://kilkakon.com/. It is therefore better to first make and test your shimeji with Shimeji-ee, and only try using it with Shijima once you've confirmed that it works as expected.
If you need help with making your own shimeji or if you're having trouble using Shimeji-ee, you can join Kilkakon's Discord server. There's a channel dedicated to shimeji discussion and the people there are generally very helpful.






