iPad Mini 1st Gen (Jailbroken), want to upgrade from 7.1.2 > 9.0.2, not 7.1.2 > 9.3.5, don't know where to start.
The short answer is, you can't.

First, for the most part, using SHSH blobs to install an unsigned version of iOS hasn't worked since iOS 6. There have been some recent developments with a tool called Prometheus, but it will only work on more recent versions — at the moment, that basically means iOS 10, though I am greatly simplifying the situation.

Second, though it's a moot point, you would need to already have saved the SHSH blobs for 9.0.2 while it was being signed/while your device was running 9.0.2.