To make a solarization, make a print or expose film as usual. Re-expose it to the room light for a short instant. High contrast paper seems to work well for solarizations. The enlarger light, without a negative in the carrier, may be used instead of the room light. For color solarizations, on color paper or film, use different color filters in the enlarger, or tape filters or gels, or even cellophane, over the room light, to get different colors on the final solarized image.
Solarizations can be unpredictable.
Fake solarizations and posterizations are ways to achieve similar effects consistently. To make a fake solarization, make a high contrast positive and a high contrast negative of the original negative. Then sandwich the original negative with the negative-positive negative, and contact print that onto a piece of continuous tone film. The resulting print from the continuous tone negative looks like a solarization. Not all the above steps are necessary.