First, basic historicity should be largely assumed but 'literalism' should be eschewed whenever nuanced distinctions may be practically helpful for reconstruction's sake. In other words, be neither defensive nor critical for theo/ideological reasons. If we're going to be trusting or skeptical, both should only apply in the interest of furthering historiographical objectives. We aren't trying to shore up our camp, here. We're trying to analyze the Gospel's content with greater historical sensibility.
Second, causal factors must be held in tension with theological humility. Each interpreter has their own philosophy of History, and their own philosophy of God's involvement with History, but we must accept that Jesus at various moments acted, reacted and was acted-upon, and that divine power was neither absent from nor dominant over the recorded events, practically speaking, from what we can tell. In other words, as events actually unfolded, the Father and Son were precisely, and only, two of our players. History's stage must respect all the dynamic personae.
Third, we must draw careful distinctions about what our finished project will or won't claim to be, and thus sidestep traditional fears of constructing this summarized narrative. Far from producing a Tatian-esque textual rearrangement, we'll not craft a remastered medley of four separate tunes. Instead, we'll compose in our own words a song that is technically new, but which succeeds at three tasks: 1) to faithfully capture the spirit and soul of our source texts, 2) to represent both Gospel content and contextual 'background material' holistically, and 3) to provide greater awareness and insight into aspects of texts that we should already know, but often fail to recognize.
Fourth, the purpose of writing this Gospel Based History is not to discover something the Gospels didn't already tell us, but to build upon and make more from what they actually do tell us. In other words, reconstructing sound History is a lot like constructing sound Theology, except that Historians naturally ask different kinds of questions. We will primarily focusing on scripture's testimonies about practical happenings and examining how persons who carried ideas and beliefs both took actions and interacted with one another, in ways that may or may not have been fully in line with anything God Himself was attempting to do, at particular times.
All in all, a Historian of Jesus' life needs to believe in the texts of the Four Gospels, but analyze those texts historically. She must read, consider and comment on them while asking different sorts of questions than theologians typically ask. She must write different sorts of overviews than theologians have usually written. Like any good Theologian, she must build up and make more of scripture's God-breathed content, in ways that neither add to nor take away from scripture's claims, but which enhance what is already contained there. The Historian must engage with historical issues without ignoring theological truths, and construct narrative summaries without ignoring the deep perspectival distinctions of Matthew, Mark, Luke and John.
Like any good work of Theology, a Gospel Based History should impact readers by making them *more* eager to dive into the scriptures, not less. Such a project will produce neither a radical new vision nor a regurgitation of church tradition, but a fresh four-dimensional (ie, fully spatial & temporal) perspective on the original Gospel, and especially on the most living and active aspects of that Holy Gospel. The goal of such work will be merely to bring out the full life of the One Story we find in the four irreplaceable Gospels.