If you already have Steam installed, then all you need to do is to click on their respective links.
To promote the upcoming game, you can now play the Half-Life, Half-Life 2, Half-Life 2: Episode One, and Half-Life 2: Episode Two games for free on Steam until March 2020 when Half-Life: Alyx is released.
Steam users received a notification earlier announcing the Half-Life promotion, but the notification appeared to have gone out too early because it soon disappeared. Numbers have been climbing across the series in the weeks since the announcement of the new game, but Half-Life now has more people playing it than at any time in the last seven and a half years.
Valve's blog post indicates that the Half-Life Collection will only be free-to-play for a limited time - until Half-Life: Alyx is released.
Still, the games are all available.
They're making it as easy as possible by giving the games away for free until Half-Life: Alyx's release in March.
Valve says Half-Life: Alyx was "designed from the ground up" for Virtual Reality and "features all of the hallmarks of a classic Half-Life game", including "world exploration, puzzle solving, visceral combat, and an intricately woven story that connects it all with the characters iconic to the Half-Life universe".
I doubt there's many gamers left who haven't experienced Half-Life in one form or another, and even if you have, Valve is also offering some steep discounts on owning the Half-Life Complete bundle, Half-Life 1 Anthology, and even the Valve Complete Pack, which you may want to take advantage of. Fan projects such as Black Mesa and Hunt Down the Freeman do not get in on the free fun since since they're, you know, unofficial. You can pre-order it now for £41.84 instead of the standard price of £46.49.