if you have not setup your sd card (for those new to a2sd):
A. backup your sd card to your computer. can't stress this enough, as partitioning will wipe the entire card.
B. Reboot into recovery
c. In android system recovery, select partition sdcard 
d. In the partition sdcard menu, select partition sd
e. Follow the instructions on the phone to partition your sd card. You can optionally set a swap file while you are partitioning your sd card.
F. In the partition sdcard menu, select sd:ext2 to ext3. Follow the instructions on the screen. 
G. only perform this step if you want ext4 and your rom supports it: in the partition sdcard menu, select sd:ext3 to ext4. Follow the instructions on the sdcard.
h. Press the back button on the phone to go back to the main menu.
I. Put the a2sd file on the sd card (and all your other files you backed up earlier) by selecting usb-ms toggle and transfer the files to your computer.