Secure Notepad is a simple TXT editor, just like good ol' Windows Notepad. I would say they're identical, so it won't take you long to learn how to use it. The great difference is that Secure Notepad encrypts your text files saving them in a native ETXT format.
ETXT files can only be opened with Secure Notepad, and only if you enter the correct passphrase. You have also the option to use a file as a key to grant access to the file. As another security feature, Secure Notepad gradually fades to white the text whenever its window is not active, hiding it from prying eyes until it finally minimizes the window automatically.
What I missed is the possibility to choose between different encryption methods or at least have some information about the level of security, such as the program telling you how strong your selected password is.