Connecting balanced low impedance output with unbalanced high impedance input
the connector selection: If your guitar pedal already exists, then it probably has TS unbalanced jack for normal guitar cable. There's nothing selectable. If you are going to build something, use the same, because then you can some day connect a guitar, if needed.

Your audio interface can output a little too much for normal guitar compatible input, causes overdrive distortion. In that case insert a 2 resistor voltage divider to attenuate the extra level off. You can well make a clearly marked special cable which has the needed resistors inside the connector.

The balanced output of the audio interface has 3 wires: hot(+). cold(-) and GND. If there happens to be an output transformer, you must connect differently than if there's not. The interface manual SHOULD tell what's the right connection to unbalanced input. My guess is connect hot(+) to the cable wire and connect GND to the cable shield. But it's as well possible that the manual orders you to have 2 wire+shield cable and connect one wire between hot(+) and the plug tip, another wire 
between cold(-) ad the plug sleeve. The cable shield should be connected either to output GND or also to plug sleeve. As said: See the original interface manual!