The same way Sylvia Browne or Jean Dixon make predictions. Just say a bunch of vague things that could possibly happen (e.g., two powerful countries will go to war) and some of them are likely to come true. The rest are forgotten.
Also, it helps that the New Testament writers were aware of prophesies of the Old Testament and so they could alter their stories to fit them (like how they had to get really creative to have Jesus born in Bethlehem.)
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