You want to build a trs to rca cable i assume. If you already have a trs end wired you need to run the ring and shield/ Sleeve to ground on the rca end... If not you need to run tip to tip and jumper the ring and shield/ sleeve together and run the shield to shield:
This is the best pictures I could find .. For your purposses You really want the first pictures right half as an rca and the for the right half of the cable and the right half of the second cable as the left half of your cable:
1) the 1/4" in the first drawing is the rca...
2) the tip is the red wire and the ring is the blue
3) the first picture on the xlr side highlights the jumpering comment
Here is the picture of the trs 1/4": note the tip is the tip( typically red wire) the next band down is the ring.. Typically blue wire or clear.. The next band down is the sleeve/shield or ground conductor it is typically a bare drain wire that is coincident with the copper braid that holds the whole cable together directly below the outside rubber jacket..
Hope this helps
A