Cross Processing is when you put a film though a chemistry that it was not designed for i.e E-6 reversal film though a C-41 Negative developer.
It is a very easy way to get "wacky" colours and extreme contrast and suits a wide range of subjects.
I have found that the best way to do this is to buy cheap out of date slide film, as colour fidelity isn't an issue here and then find a willing minilab for process only.
Be sure to inform the Lab that you want it to go though the wrong process as a helpful lab assistant may just send it off for E-6 process!
When you get your negatives back they will look like this:
After printing or scanning the positive image will probably look very strange, high contrast and normally quite yellow or green (the unpredictability is part of the fun):
Depending on your subject you might like to leave it just like that, but in the above picture I felt that green skin tone was not that flattering.
So for the final rendition after scanning I sent the image to Photoshop to 'tweak' it in curves in order to give slightly better skin tones and increase the contrast. There is really no correct rendition of a cross processed image, but that's one of the things that makes it fun, you could even do a series all with different hue adjustments.
All Images and text ©Mark Antony Smith 2007