How to do halftones

Ive been reading up on a Process called halftones,
What is a Half tone?

Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size or in spacing. 'Halftone' can also be used to refer specifically to the image that is produced by this process.
Where continuous tone imagery (film photography, for example) contains an infinite range of colors or greys, the halftone process reduces visual reproductions to a binary image that is printed with only one color of ink. This binary reproduction relies on a basic optical illusion—that these tiny halftone dots are blended into smooth tones by the human eye.

I'll be creating halftones in photoshop so i think its time i should learn how it is achieved. time to watch loads of youtube video tutorials, these really are a great way to learn thumbs up to the 21st century.