you dont need a transformer.
just "unbalance" the cable.
on the XLR end, pin 1 (ground) will be "jumped" to pin 3 (which is normaly OOP - signal).
the resulting miniplug end can be wired the exact same way, w/the shield grounded. End result is an unbalanced mono 1/8th plug.
viola!
If you find noise on this, then wire the XLR end normally and then ground the mini plug.
Now, if you are making a stereo cable here.
then you ground at the XLRs as described, and then wire the 1/8th mini correctly w/the tip being left, the ring being right and the shield is grounded.