There are a number of references linked through the Resources page on the Worldwide Pinhole Photography Day site. There are many paths to get there ranging from crude simple modified cans and boxes up through units based on regular cameras. Depending on the type of camera, one can make a special lensboard or a modified body cap.

I've dabbled in the stuff myself.

Be it also noted -- WPPD is April 29th this year!.

DaveT