Connecting a DIY guitar pedal to a pedalboards power supply
As a fellow musician, I applaud your effort, but must point out that the nature of your question leads me to believe you need a lot more information before attempting to build an electronic device.

A power supply (I assume you have a typical AC/DC transformer or "wall wart") and a battery are both voltage sources. As long as the battery or power supply can provide the required current and the voltage is correct, they are interchangeable.

But to address your question more directly, a guitar pedal (at least the ones I have) typically have a "DC barrel jack" which has a negative polarity. In other words, the polarity is the opposite of many other DC devices.



The connector on your power supply provides 9 volts, but the outer contact will be positive while the center pin will be negative.



You can see on a 9V battery that the smaller "male" terminal is positive. As long as the positive (outer) terminal of the power supply connects to the same power input pin on the device as the positive terminal of the battery, the polarity will be correct.