How does one go about making items and having them appear as actual site items?

There's usually new avatar items each month available just for that month that can be bought with diamonds. Of those items, at least the held items can be made into custom versions.

Here's the link to October's avatar items. https://www.sylestia.com/forums/?thread=29560

Scroll to the part about ordering custom items and it gives a lot of info there on what to do.

You will also need an art program to make a custom item like photoshop or gimp.

Once you have your custom avatar item, you'll always be able to order extras for 100 diamonds each.

Sorry, I realized I forgot to ping. I hope this helps but if you have any other questions I'll try to help.