To reset the Samsung Galaxy Ace II:
Make sure phone is at least 50% charged or more.
Remove Sim and SD cards.
Turn phone off.
Press and hold Power, Home and The Center of the Volume Up/Down Rocker buttons.
When the screen lights up from Black to Light Grey...release the Power, then Home, then Volume Rocker buttons in that order.
You should now be on the recovery screen and can use the volume rocker to navigate down a couple of lines to "wipe data/factory reset", then confirm using the Power button.
You will then be informed that this will delete everything on the phone and asked if you wish to continue.
Use the Volume Rocker to navigate down 5 or 6 lines and use the Power button to select Yes.
Wait until format is complete, then select reboot system, again using the Volume Rocker button to navigate and the Power button to confirm.