Link2sd is an app that uses a second partition on the sd card to link your apps to it, this means it indicates to the phone that it has a larger memory so you can install as many applications you want.
Requiriments
Rooted OT 990.
ClockworkMod Recovery.
Formatted SD card (make a backup of your data before formatting).
Steps
Reboot your phone into recovery mode.
In the recovery go to: Advanced -> Partition SD Card.
Select the size of the new partition (Link2SD partition) and select ext2... I selected 512 and I've never filled it.
After you've formatted the SD card, reboot your phone.