In the simplest setup you get a calibrated mic like the umik-1 (about £100 plus a cheap mic stand from eBay/Amazon), which is USB, and you connect to your computer. You then run a free program called REW, which outputs sound ‘sweeps’ via USB to your DAC, while the static mic simultaneously measures the response (which REW captures).
You do this multiple times - for your hero position, plus others around it, and you save them all as a REW file containing the measurements.
The sweeps are varied, but generally people use a log-sweep which goes from low frequency to high frequency. As the curve is known, what the mic records shows how the sound is distorted from this known curve due to your speakers and room interaction - in all bar the most perfect dedicated and treated listening room or sound studio, what you get back will be quite different to what REW sent the DAC. In fact I was amazed how bad mine was when I first saw it, but most people’s graphs show the same - the sound’s all over the place.
There’s a lot if info out there which will be helpful to read but not really necessary - if you want to give it a go there are a few guides here on this forum and plenty of people can walk you through it. It’s not rocket science but it’s worth taking time and doing a few practice runs.
If your room isn’t perfect, and you can’t (or don’t want to) treat it acoustically, you’ll probably benefit. For many people - like me- with hifi in a domestic setup it’s a far from ideal room, the benefits are huge, taking from a system I dont like listening to, to one I really do.