Seasonally adjusted unemployment data

How exactly do they do the seasonal adjustment when they say seasonally adjusted jobless claims. I get why they need an adjustment, but what exactly am I looking at? Do they start with the real number and then subtract out the unemployment claims they think would normally happen for seasonal shifts during that period, or what? I have always wondered this.

Surprising hard to google. All the answers just tell me why seasonal adjustment is important but not what the adjustment is.

My google-fu turned up:

It also turned up the following book, which based on the table of contents seems to walk through a lot of those ideas – though it could be an inscrutable mess. Sadly my curiosity isn’t enough to justify $130.