Then connect the fan wires with the power adaptor output wires.
The polarity MATTERS, so you might switch the wires if the fan doesn´t turn.
On some adaptors you can change the polarity with a switch on the adaptor itself.
Once you have connected the wires correctly, isolate both wires with electrical tape so they don´t touch each other to prevent the adaptor from causing a short cut. This could destroy the adaptor or in the worst case cause a fire. Better be safe than sorry!
You can adjust the fan speed by adjusting the output voltage of the adaptor.