My print is warping: how can I ensure that it cools evenly? My print is warping and I suspect it is because of uneven cooling.

How can ensure my print cools evenly? Should I put a fan on the bed? Will no fan and a tall skirt help?

Or are my edges curling up for another reason?
Without more information we cannot address what is causing your edges to turn up. This can depend on the model being printed, the process, the material, the bed, and the method used to bond it to the bed.

As far as the initial question, a fan will most certainly ensure uneven cooling.

The middle of the print is, and will remain, the warmest.  If you want the print to cool evenly, you'll need an enclosure to block out drafts and air currents, and you'll need to bring the heated bed temperature and enclosure temperature down slowly over a long period of time.