Instructions
Complete all the "Case Files" missions in the original 72 hour mode until you are asked to go back to Carlito's hideout at 10 a.m. on the last day. Saving survivors that aren't listed in your case files is optional, but it won't affect the ending either way.
Go to Carlito's hideout by 10 a.m. on the last day. This is located in the northern end of the mall and will be clearly displayed on the map. Talk to Isabella after the cutscene ends.
Go back to your hideout. Go up to the roof and be standing on the helipad when the clock strikes noon. Another cutscene will occur, and when it's over you'll be in overtime mode.