Research the Classics
The best place to begin is by researching invocations in epic tales. By reading and comparing the classics, you can begin to see patterns in the language and structure of the invocation. Summarize what information transpires in the invocation, and look for similarities. For example, in "The Iliad" and "The Odyssey," both by the poet Homer, the first lines begin with praying to muses. Note any descriptions of setting, characters, or gods and what adjectives are used to describe important players in the epic. In the opening lines of "The Aeneid," Virgil ascribes characteristics of Aeneas by using words like brave and just.
Plan the Story
Before you can write an invocation, you have to know the plot, characters, setting and other key components. An epic poem follows the quest of an extraordinary hero who faces grave trials and gruesome foes. Brainstorm those characteristics of your epic hero that deserve admonishment. Create a map of your story outlining the challenges your hero will face on the journey. Your legend should have many exotic and fantastical locations to capture the full epic effect. Jot down brief descriptions of the obstacles and settings to incorporate into your invocation.
Call the Gods
Now you are ready to begin your invocation. Ask the god(s) to bless the retelling of your epic. Ancient epics were recited to an audience, and they believed the god(s) would go into the poets to tell the story. It is common to identify the gods by their supernatural powers and other spectacular qualities. This step is to pay homage and respect to the god(s). You can use traditional deities or invent your own.
Invoke the Muse
The calling of the muse is the crux of the invocation. Muses were goddesses of inspiration; and artists, poets and musicians would invoke them to bless their craft. In your invocation, praise the muse for her creative influence. There is even a muse of epic poetry in the Greek tradition, but feel free to make up your own to suit your purposes.
Introduce the Hero
Now that the divine has consecrated your epic, it is time to introduce your audience to the hero of the tale. Use lofty and descriptive words to establish the renowned attributes of your hero. Ascribe epithets -- grandiose adjectives used as names -- for your hero, such as relaying his or her parentage or past triumphs. For example, Odysseus is repeatedly referred to as Laertes' son in "The Odyssey." Epithets are established in the invocation and repeated throughout the story, so you need to clearly build your hero at the epic's beginning.
Summarize the Story
The final stage of the invocation is used to hook the audience's interest by giving them a taste of what is to come later in the tale. Epics typically start in medias res (in the middle of things), so you must first clue in your audience to what has happened so far. Then give a brief outline of the daring feats, Herculean tasks, unfamiliar lands and hideous monsters the hero must face throughout his or her journey. Present your audience insights into the hero's quest and themes developed throughout your story. This provides a solid introduction to your story. With all these elements present, the invocation is complete.