You've probably heard this Sunday School humor tidbit:
Sunday School teacher holds up a picture and asks the class, "What is this?"
Little Johnny answers, tentatively, "Well, it looks like a squirrel, but I know the answer is 'Jesus'."
I can laugh at the Little Johnny and the Squirrel story, but I think it's true too. The best teaching and preaching always makes the answer "Jesus."
Not every biblical text is explicitly about Jesus of course. But no matter what it looks like, we can show that the answer is Jesus.
Here's how I approach biblical texts in the mode of gospel-centrality:
If I'm looking at an exhortation/command/Law, I ask what precipitates it. Sometimes you have to draw in the gospel reminder if it's not immediately in the text or context. For instance: Leviticus is chock-full of commands, but this book comes after Exodus, after the Israelites are set free from Egyptian bondage and are in the wilderness. So I remind myself and my church that obedience is a response to God's freedom, not the leverage for God's freedom. In other words, we don't obey to be set free; we obey *because* we've been set free. In the same way Jesus announces the blessings of the kingdom coming in the Beatitudes, and then proceeds to tell us what life in the kingdom looks like (the rest of the Sermon on the Mount). Pronouncement precedes exhortation; being precedes doing.
This is easier to do in Paul's letters, because Paul is always connecting commands to gospel pronouncements, couching what we do in "what we are." One has to try really hard to divorce Paul's exhortations from Paul's gospel proclamations. A lot of preachers do it, but you really have to put the blinders on. It gets harder in the Old Testament, but even in some of the hard core hellfire and brimstone passages of the Minor Prophets, there are plenty of little gospel pronouncements. (Malachi's burning furnace and threat of God smearing dung on our faces comes after he explicitly reminds us "I have loved you.")
If the text I'm looking at is a story of some kind, the most important thing I try to do is use it to point to Jesus as the hero of history. So David and Goliath becomes not about our having courage in the face of adversity but about Jesus defeating sin/death/Satan on our behalf. We aren't David in that story; we are the scared Israelites.
A good template for gospel-centered biblical storytelling is Ferguson's "Jesus is the true and better __________."
This is extremely important. And once we make it our routine practice, it will get easier to see the gospel springs running beneath the hard soil of God's harder words. Once we train our eyes to see it, we will see the gospel of Christ crucified and resurrected as the theme of all of Scripture, not just the New Testament, and not just the parts in the New Testament that are "easy."
Eventually we can look at any text and say, "Well, it looks like a squirrel -- and maybe it is a squirrel -- but we know the answer is Jesus."