To create it, you need to visit Chaos Goblin and select Regular Combination in the dialogue with him. Next, you need to move Devil's Key (+1 - +7), Devil's Eye (+1 - +7) and Jewel of Chaos to the appeared window, and also have the required amount of zen (Required zen in the window for combining). Then you need to click Combining.
If you're lucky, you'll get an Devil Square Invitation of the same level as the ingredients used (Devil's Key, Devil's Eye). The ingredients and zen will disappear.
In case of failure, all the ingredients, as well as the required amount of zen, will disappear.