So in the midst of combat with the Kyubi, Minato...
1) Summons the death god.
2) Captures the Kyubi's soul.
3) Redirects the vessel for the soul to Naruto.
4) Tailors the seal to leak chakra in a controlled manner to Naruto for his protection.
5) Injects some of his own chakra into the seal to trigger a message when the seal is weakened.
6) Injects some of Kurina's chakra into the seal to trigger when Naruto is battling Kyubi.

I don't think we'll ever physically see a flashback of this sealing process, as it has become far too ridiculous at this point.