Thanks for buying this extension!

Only a few things to know - look inside the script safe_device_id() and read the comments there. 
If you need to use HTML5 or non-ascii characters you should call safe_save_htmlsafemode() before you do anything.
The downside to this is (largely) increased filesize. HTML5 and non-HTML5 saves are NOT compatable.

The default encryption used by this extension is an RC4 Cipher. There is also support for much the stronger cryptographic standard of AES. AES GML is available through a seperate package available at https://marketplace.yoyogames.com/assets/5093/aes-gml

To enable AES encryption, follow the instructions in the script safe_save_use_aes()