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 2: Strip the Wires
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.
Step 4: Attach the Wire
Bend one of the tabs over the exposed wire. Make sure you have a good mechanical connection. If the wire is loose, it will fall out.
You may want to bend over the other tab, but if you decide to use the clip again, you may be able to salvage the clip and use the other tab. Just make sure you have a good connection.
Step 5: Slip the Boot Over the Clip
Slide the rubber boot over the gator clip.
Do the same to the other clip.
Step 6: Test the Leads
Put the multimeter on the appriate setting for the value you think it is.
Put the clips on the meter.
Check the meter's reading.
If you pay attention to the negative symbol, you can see what the polarity of the wires is.
Label the leads with masking tape, especially if you don't have different colored boots.
Step 7: Glory in Your Completion
You can use it to power projects. If your project is fussy about what voltage you need, you may need to build a control circuit.
If you are just powering something that needs to go, like a motor or something like that, then you should be able to just hook the gator clips to the project and get going.
Since this is a wall transformer, it will continue to draw power even if it is not being used. Unplug it to when not in use to conserve energy.
Have some fun!