Screen printing is a process suited for printing on unconventional surfaces, such as clothing or outdoor signs. You won't find much screen printing done on paper, unless of course you look at the work of Andy Warhol or the poster art of Frank Kozik. Generally, paper is the domain of the four-colour process, or CMYK printing, which creates very precise results. Screen printing is far more rough and ready.
When a design is colour separated for screen printing, a film positive (rather than a film negative as is used in photography) is created for each colour. Areas of 100 per cent black will reproduce as 100 per cent of that colour, and coarse halftone screens are used to create percentages of those colours. Each positive is used to create a separate screen, which ink will be forced through with a squeegee on to the desired surface. Usually all screens are crudely registered on a carousel-like device and ink is applied to the surface, one colour at a time.
In this tutorial you'll use Alpha Channels in Photoshop as your screens - scanning a hand-drawn positive and placing it into each new channel. A selection will be generated and then filled with colour on a new layer. To make your channels behave, you'll need to alter the default settings in Photoshop first. So, as you get to the point where you create your first Alpha Channel, double-click it in the Channels palette. When the Channel Options appear, change the Colour Indication setting so that colour indicates selected areas rather than masked areas. By doing this, you'll be able to generate selections without having to invert them each time.