The impulse response will always go after the amplifier, just like a speaker on a combo or cabinet when paired with an amplifier head. There are a couple of ways you can go about using an impulse response, however.
One of the most common ways is to use an IR loader in your DAW. You assign your guitar VST of choice to a track, bypass the cabinet section if there is one, then put the IR Loader next to simulate a cabinet. There are loads of free and paid options for IR loaders if you’re going to go this route. Some guitar plugins also have their own IR loader section, allowing you to import your sounds and use them within the plugin itself.
The second option is to use a hardware unit like the Neural DSP Quad Cortex or Line 6 Helix units. You can import your impulse responses into these units to be used live on stage, allowing you to take your well-crafted tone wherever you want with a minimum of fuss. The way you do this will depend upon your particular unit, but this will be covered somewhere in the manual.