8
Check the XOOPS core files. See, how the kernel/user.php, database class etc. work and see, what options are available to you.
Get a module (WF-Sections for example) and look, how that works.
If you feel like writing HTML code in your .php document. Stop!. Make a SMARTY template for it instead (if it's an advantage). Write your code object oriented. Read the xoops_version.php of e.g. news module and use everything you want from that.
If you do that, I'm sure you will have a good module, easy to read and understand and easily fulfilling the requirements for a certification.
Good luck and have fun.