Configure your AirPort Express to create a new network via the tutorial here or to extend an existing network via the tutorial here.
Connect your printer to the AirPort Express via USB cable.
Launch AirPort Utility from your Applications:Utilities folder.
Select your AirPort Express from the list on the left and confirm that it has detected your USB Printer.
Select Preferences from the Apple Menu at the top left of your screen.
Select Print & Fax from the Preferences window
Check the bottom of the Print & Fax window to see if your settings have been locked. If the settings are locked click the lock.
You will be asked to enter an administrator username and password. Do so and click the OK button.
Click the + button on the left side of the window to add a new printer.
Your printer will appear under the Default tab. Select it then click the Add button.
Your printer will now appear in the Print & Fax window and is ready for use!