From SU's guide
If I wanted to do it, is there a step by step process?
Assuming you are rooted;
1) Backup your SD card contents to your PC
2) Download GParted Live cd, burn to CD and boot to it GParted -- Live CD/USB/PXE/HD
3) Connect your device in Recovery mode and mount as USB mass storage
4) Resize the existing FAT32 partition from the back, leaving space after it and create an EXT partition in the space. Steps for the GUI are below.
In the drop down menu (top right) select the SD card (be careful not to select your HDD)
Right click on the FAT32 Partition and select resize
Press the Up arrow on "Free Space Following" until you have the desired sizes
Click Resize
right click in unallocated space and select New> EXT format of your choice
Click Apply
5) now you just need to flash your ROM with a2SD+ support.
Alternatively, you can download and ROM manager to partition the SD card, however you cannot easily check the partition is correct. Also, I do not recommend using ROM manager at all. It looks easy to use (and it is) but people using this can get into trouble and it takes away the requirement of knowledge on what it is doing, which is useful to help you get out of trouble.
Guide here:
http://androidforums.com/desire-all-things-root/220627-faq-apps2sd-updated-19-03-11-a.html
Gparted here:
GParted -- Live CD/USB/PXE/HD