In Canto I, Dante, lost in the woods, wants to get to heaven, but his way is blocked by a wolf, a leopard and a lion, which represent his sin. The only way up to heaven is down through hell, the place where we confront our own demons. Suffering through the downward journey is the way up to heaven. In the poem, the geography of hell is literally downward through different, progressively worse circles, but that downward journey is also a metaphor or symbol of the necessary, if painful, journey into the depths of our souls. We have probably all heard the phrase hitting rock bottom, as in an alcoholic having to lose everything, becoming the first step on the route back up to a better life.
In the ninth circle of hell, Dante and Virgil have to climb down Satan's fur to his bellybutton or naval, where they find the earth's center and can begin the upward climb to heaven. After they pass through Purgatory, Dante and Virgil begin the upward journey.
Some passages that might help get you to six lines of poetry, and you should look for more as this is only a pointer, are as follows. Also, I am probably not using the same translation as you, so you will have to find the corresponding passages:
In the introduction, Dante first tries the direct approach to heaven and finds that "not far from where the ascent began" a leopard blocks his path. This symbolizes that from the beginning he will have to go down to go up.
The leopard, wolf and lion "pushed me, step by step, back ... ." This symbolizes that the journey up to heaven is hard, not easy, for you must first go down to go up. Sin, represented by animals, forces us down.
Virgil emerges and tells Dante he must go through "an eternal place, where thou shalt hear the shrieks of hopelessness ...." This represents that Virgil in his wisdom knows that you must encounter pain to understand the blessings of heaven.
When Beatrice in the beginning urges Dante on his journey, he writes:
"as little flowers by the chill of night/ bowed down, when brightened by the sun, stand all erect on their stems; so likewise with wearied strength did I ... "
This metaphor of the flower bowed down, then rising up to the sun is very important as representing Dante and his journey: like the flower, he goes from down to up. He has to be down to ask the questions that will lead him up. He is as fragile as a flower and as dependent on the light (God) for nourishment as a flower is on the sun.
After hitting rock bottom, Dante and Virgil at the very end of the Inferno head up: he writes that he and Virgil "in order to regain the world of light," enter a "dark and hidden path, and ... Went up... And saw again the stars." This once again represents that the path to heaven is not straightfoward or easy to find, but that you must first go down, encounter Satan's ugliness, and then you can head up toward the stars, which symbolize heaven.
In the Paradiso, canto 3, lines 61-96, Beatrice teaches Dante more about down being up. People in heaven achieve heavenly joy by not aspiring to keep moving upward. Dante asks "do you wish for a higher place, to see further" and she replies, "the power of love quiets our will, and makes us long only for what we have." If appetite leads those wanting to move "up" down to hell, love leads those who are humble up to heaven.