I have to develop an app for a specific version of iOS (I am just a freelancer, doing what I am told).
How I install iOS 8.2, or below (8.1, 7.x are fine too) on an iPhone 4S?
There's no official way of installing an older iOS on a device that Apple has stopped signing the SHSH keys for.
Your best bet is either to get a hold of a device that still has an older iOS or just using a simulator when developing the app.
Short answer, you don't.
Apple will only sign the current OS, or in the case of a device incapable of running the current OS, the latest it supports.
There are no models that support iOS 8 that don't also support up to 8.4.1 [& even the coming iOS 9]