Nice and easy:
1. Make sure that your card's firmware supports WPA. Go here or here, for info and files.
2. Get the wpa_supplicant installed on your machine.
3. Modify /etc/wpa_supplicant/wpa_supplicant.conf with your network settings:
4. Launch the supplicant and you are done!
wpa_supplicant -iwlan0 -Dhostap -c /etc/wpa_supplicant/wpa_supplicant.conf