You will need some of these:
power supply (go with a junker)
Alligator clips with plastic boots
wire strippers or cutters
about 10 minutes
You will be interested to know what the output values are. This one listed 9 volts DC as the output with 600 miliamps.
The label will also tell what the polarity is. This one has a negative on the outside, positive on the inside.
It also indicated that it came from a telephone. I think it was from the dump. Nobody will cry if this is gone. You should check with any potential owner before you perform this surgery on a wall transformer.
Step 1: Cut off the output plug
Cut the wires.
Cut one wire shorter than the other. It is nice if the negative is shorter, but not essential. Mostly, you want to make it so that the two tips cannot easily touch each other. If they contact, you will probably blow the fuse, which you won't want to replace, and will probably cause you to toss the transformer.
Step 3: Put the boots on
If you forget to to put the boots on, you can get them on, but it is a pain.