Please, READ first before attempting if you are a noob, it’ll save you some gray(grey) hair. I am in no way responsible for any issues you have for following this guide, use at your own risk.
WARNING: This will erase EVERYTHING on your SD card so please transfer the contents of your SD card to a computer or external hard drive PRIOR TO starting this procedure.
1. Boot into recovery (hopefully you have amonRA v2.3)
2. Select "Partition SD Card". You will be prompted to set an amount for the SWAP, ext2 and FAT.
3. Enter zero for the SWAP size, 1024 or 2048 for ext2 and the rest for FAT*
4. After the process finishes, select "Upgrade ext2 to ext3"
5. When the process is complete, go to the wipe menu and wipe EVERYTHING in it.
6. From the main menu, select USB-MS (your phone should be connected to your PC) and copy the Synergy ROM(or your ROM of choice) to the root of your SD Card.**
7. Flash Synergy(or your ROM of choice)
8. Be patient at initial bootup because it can be quite long.
9. I recommend that your 3rd party apps be installed freshly from the market initially. They will automatically be place in the ext3 partition. You can verify it by checking to see that they are listed in the /system/sd/ folder which itself contains 3 folders: app, app-private & dalvik-cache.
After you have followed the directions to partition your card, followed by a flash of the rom and installation of your 3rd party apps & games, go to Menu-Settings-SD & phone storage and check your available internal storage space. If it is 300+, it is safe to assume that the built-in a2sd is working. Furthermore you will need a system level file manager to see the /system/sd/ folder. The Astro File Manager cannot see it.
*These sizes are only used for reference, but excellent choices. You may want to consider a swap partition, even though the stock HTC kernel does not support it. Future kernel development may take advantage of swap and doing so now can save the hassle of re-partitioning later. Food for thought.
**This guide was done while using THE Project Synergy ROM, however it should work across the board.
Hope this helps.
Special thanks to dougjamal, lowetax and all the Mynions helping out around the world (We Global!)