Two classics of the Voyage and Return form are Alice in Wonderland/Through the Looking Glass and The Lion the Witch and the Wardrobe. Both have examples of the 'portal' which is common to many of these stories - the route to the other world. Rabbit hole, mirror, wardrobe - they mark the end of this world and the beginning of the next. In The Wizard of Oz, the tornado is the portal, blowing Dorothy's house over the rainbow. Once the portal has done its work 'we're not in Kansas any more'. Anything can happen - reality as we know it has been left behind.
But the stories don't have to be fantastical - the protagonist can enter another world in social or geographical terms, as Paul Pennyfeather does in Decline and Fall, or Harry Lime does in The Third Man.
The hero or heroine of such tales can be dreamy and unfocused or naive and open to experience. They can be generic - an adventurer, up for anything. But they do need to begin with a set of assumptions that shape their world, and in the course of the story these assumptions do have to be challenged or undermined in some way. A brilliant example of the form is another Greene novel, Travels with my Aunt, in which the narrator returns home in a way which is unexpected but satisfying. (I won't say how in case you haven't read it yet, and it's a brilliant story, clever, vivid and a real page-turner.)
At some point the new world becomes more dangerous and sinister that it is exotic and exciting. A crisis of some kind will take place, and the narrator will be in peril. Mr McGregor might be chasing Peter Rabbit to put him in a pie; the Red Queen might be threatening Alice with decapitation. Essentially, at this point they change, they expand their knowledge and awareness. Innocence is lost. The Voyage and Return plot is a plot of self discovery and 'coming of age'.
Christopher Booker (our plot guru) sums up the stages like this: 1. Anticipation Stage and 'fall' into the other world; 2. Initial fascination or 'dream' stage; 3. Frustration stage; 4. Nightmare stage and 5. Thrilling Escape and Return. Top tip - if you take one of the novels I've mentioned, you could break the action down under these headings. Or try it with another novel which seems to fit this story genre.