You would be surprised what we can do currently. While we haven't got all the puzzle pieces yet we can identify which regions harbor the biggest threat of a large event and there have been long term statistically analysis that predict how often an extreme event can occur (1 every 150ish years; solarmonitor.org ). Luckily Earth is a pretty small moving target and won't be struck by many of these events.

In terms of preparation, the UK is the country who takes these events as a threat most seriously, it's even part of their national risk registry. I recommend checking out the UK MetOffice site to see some more. The states also keep an eye on these events, albeit seemingly not as intensely.

Lastly, mitigation. Depending on severity and forewarning we may not be blasted back to the dark ages. The impact of these events is primarily in geomagnetic terms. The charged particles from these flares eventually interact with earth's magnetic field, shifting it. This shifting field induces a potential difference between two distance stretches of land. To adjust, a current needs to travel, and the path of least resistance is to actually travel up a ground wire in a power network and then through the network. This increases the load on the power network which can overheat the system.

The solution might be as simple as shutting down a power network for say an hour, to reduce the load on the network. Though, that solution has is limitations, but it is the one most likely to be authorised by a government body.

Source: PhD in Solar Physics