Convention for Tip Ring Sleeve connectors whether they are 1/8" or 1/4" is
Tip = Left
Ring = Right
Sleeve = Ground
from this you go to your xlr's.
FXLR (left)
Pin 1 = Ground
Pin 2 = Left
Pin 3 = Ground (a jumper to pin 1)
FLXR (Right)
Pin 1 = Ground
Pin 2 = Right
Pin 3 = Ground (a jumper to pin 1)
The jumpers between 1 and 3 in the XLR convert balanced signal to unbalanced.
If your soldering skills are lacking (or in a pinch if a cable fails, like mine did on Saturday), you can also use a 2) MRCA to 1/8" cable stereo cable and RCA to FXLR adapters. These adaptors are wired the same way.