redscale is when you reverse 35mm film in your camera, ie have the backing facing the lens.
you need a changing bag or darkroom and a spare 35mm film you dont mind wasting (first time only)
i used these instructions
and shot a roll of 400asa as 100asa
it gives you a reddish colour cast that can look a bit vintage or even a bit nuclear glow. results depend on weather conditions, overcast skies being more vivid.
don't think i'd do a lot of it, just now and again. there are some far better redscale pics on flickr, check them out or give it a try yerself