The directions are found written in a book in a library. The directions are cryptic, but if you wish for know the solution, then...
[SPOILERS]
The directions are written in the Blue Book in the Chamber of Knowledge. The song "Sacred Sons Enter Sun Weathered Sands Weielding Sourcerer Slaying Scimitars"
Further hint...
[SPOILERS]
The first letter in each word S S E S W S W S S S are the directions to take through the desert. The compass on the ground near the desert entrance will orientate you (as well as the sun rising from the East)