How to Catch a White Elephant...

Go to an place where there are white elephants. Bring with you a
muffin (with raisins). Climb a tree. When the white elephant is close,
drop the muffin (with raisins) in front of it. The white elephant will
be happy, and eat the muffin (with raisins). White elephants like
muffins (with raisins). Repeat this procedure for five days in a row.

After the fifth day, the white elephant will be used to its daily muffin
(with rasins). The sixth day you climb the tree, bring with you a muffin
without rasins. Drop the muffin as usual. When the white elephant finds
out that the muffin lacks rasins, it will darken in anger.

And then you catch it the same way as an ordinary grey elephant.