Use A Vaal Orb On Any Item With Sockets.
Taken From The Offical Site -
"Vaal orbs only have a 1 in 4 chance at granting white sockets, and only a 1 in 10 chance at getting a 2nd, 3rd, 4th etc...getting 6 is a 1 in 400,000 chance when corrupting a 6 socket item."
Took Me 4 Tries To Get One But Could Take You 16 Or So in A Terrible RNG World.
Good Luck!