copied this From the xtrrom site.. this is actually really easy. After backing up your sd card, this will take you a minute
Prerequisites
1. If you have not setup your SD card (for those new to Apps2sd):
BACKUP YOUR SD CARD TO YOUR COMPUTER. Can't stress this enough, as partitioning will wipe the entire card.
2. Reboot into recovery
3. In Android System Recovery, select Partition sdcard
4. In the Partition sdcard menu, select Partition SD
5. Follow the instructions on the phone to partition your SD card. Most folks seem to recommend using the default values however you can go larger or smaller use values 256, 512,768,1024. You will also be asked to set a swap size; I stayed at 32mb (anticipate future ROM options to allow moving swap to sd)
6. Once the partition is initially created you must then convert to ext3 - In the Partition sdcard menu, select SD:ext2 to ext3. Follow the instructions on the screen.
7. Press the back button on the phone to go back to the main menu.
8. Your SD card is now ready to work with an Apps2sd supported ROM.