In this video, we're discussing some very important things related to the downgrades/upgrades to iOS 11.1.2 using SHSH2 Blobs.
Since iOS 11.3 has been released, iOS 11.2.6's days are counted. Unfortunately, iOS 11.3's SEP and Baseband are not compatible with iOS 11.1.2's and therefore, you can't use it to upgrade from a lower iOS 10 version even if you have the SHSH2 blobs saved.
iOS 11.2.6's SEP and Baseband are compatible and can be used with FutureRestore to be able to upgrade to iOS 11.1.2 with your SHSH2 blobs. Once Apple shuts down the signing window for iOS 11.2.6, it will no longer be possible so if you are planning to upgrade from iOS 10.1.1, iOS 10.2, iOS 10.2.1, iOS 10.3.1 or any iOS 10.3.x version, do it right now or you may not have a second chance.
The upgrade procedure is simple if you have the blobs. You need to patch the NVRAM with the nonce generator from your SHSH2 blobs, then you need to acquire the IPSWs for iOS 11.1.2 and iOS 11.2.6. From the iOS 11.2.6, we need the SEP and the Baseband. There are many tutorials available on the matter, and tihmstar also made a few that you can check out below.
Unfortunately, if you have the SHSH2 blobs for the iOS 11.1.2 but you are running iOS 11.2 or newer, it is not possible to downgrade because you can't patch the NVRAM. The best thing you can do: don't update further and save your SHSH2 blobs for iOS 11.2.6 and 11.3.