It was as simple as pressing a button, labelled "Generate" on the original Bitcoin client if I recall, which used your CPU to mine a block of 50 BTC. The button was removed after it started taking more than the time between blocks (10 mins) to mine a block on an average CPU.

Interesting concept that you could simply press a button to generate something that was worth at its peak a few years later US$1M. If you think that sounds like it comes from an old episode of The Twilight Zone you may be thinking of https://en.wikipedia.org/wiki/Button,_Button_(The_Twilight_Z... : "One day they receive a mysterious locked box with a button atop it ... a smartly dressed stranger ... [gives them] the key to the box and explains that if they press the button then two things will happen: they will receive $200,000..."