A migration produces two lists: the old URLs and the new ones. Pairing them correctly is the entire redirect map, and getting a pair wrong sends real visitors to the wrong page.
Interleaving them into pairs
If both lists are sorted the same way and describe the same pages, interleaving produces old-and-new pairs ready to become redirect rules.
Verify the counts before you trust anything
The two lists must be exactly the same length. A single missing row shifts every pair after it, and the resulting map looks perfectly well formed while pointing hundreds of URLs at the wrong destinations. Compare the line counts first, every time.
Spot-check both ends and the middle
Check pairs at the start, the middle and the end. An alignment error introduced partway through only shows up after the point where it happened, so checking the first few rows proves nothing.
Try it: Interleave Two Lists on SeoWolf's Notepad.