In at least 7 places: God, sin, offices, events, prophecies, themes and symbols/memorials.
God – We often fail to read the Old Testament in a Trinitarian way. Every reference to Yahweh (“LORD”) – unless we are told otherwise – speaks of the triune God. This means the divine attributes that emerge from the passage can be understood to apply to Jesus Christ. The LORD who creates, speaks, redeems and judges is Lord Jesus Christ.
Sin. Sometimes an Old Testament text is awash with depravity. It is not a misstep to see Christ as the antithesis to this. The saviour from such sin – or the judge if we don’t repent – is Christ.
Offices. This is a biggie. I think we find this almost everywhere in the Old Testament. Many “offices” or roles are fulfilled in Christ, positively or negatively. The prophets, priests and kings are the obvious ones, but don’t fail to notice how a patriarch, one of the judges or even “God’s son” Israel might point us to Christ. Remember too that David’s Psalms are much the richer when we consider that their author is the LORD’s anointed.
Events. Unfolding biblical events often reveal the gospel. This is obvious in major events like the flood, exodus and exile. Any event that illustrates salvation and judgement bring us to those main train-tracks that run all the way to the cross and the new creation. But even smaller events can reveal the character and work of Christ: the LORD passing between the animal pieces (Genesis 15), Jacob’s dream at Bethel (Genesis 28) or Joseph’s rejection of Potiphar’s wife (Genesis 39) are all Christ-revealing occasions.
Prophecies. This is the most obvious link. Sometimes an Old Testament passage is said by the New Testament to be directly fulfilled in Christ (eg. Psalm 22:1, Micah 5:2).
Themes. For want of a better word, there are some themes that run through the whole of Scripture. Examples: creation, covenant, glory, temple, presence, weakness, resurrection, persecution, cosmic conflict, kingdom, inheritance. These can all be applied in a Christ-centred way.
Objects/memorials. Places that are “named”, altars that are given a title, and even objects in the text can point us to Christ. Examples of objects include: the ark of the covenant, Aaron’s staff, the rock in the wilderness that brought forth water, the manna from heaven.
Postscript: This list is currently hanging on my wall as a little prompt in my preparation. Though I’ve put it in my own words I am hugely indebted to the likes of Clowney, Carson, Keller and Millar/Campbell for helping me see these connections.