There are all shown on your map after completing certain missions. By the time you finish the story missions, it is guaranteed that you will have unlocked all the side missions for your gaming pleasure.
There are a total of 12 side-missions to complete for this achievement. These are broken down into 4 Favours and 8 Drug Busts.
In Kennedy Town you have the following:
Drug busts
1. Cultish Ambush. Defeat 3 enemies
2. Suspicious vehicle. Investiagte the area and dispose of the car bomb by driving into the sea and get a safe distance from the explosion.
3. Park patrol – chase suspect and defeat 4 thugs
Favours:
1. Help the Rookie – follow the rookie and defeat the thugs
2. Protect the Lion Dancer – defeat the thugs and talk to the Lion Dancer
3. Lucky Money – find the money; defeat 3 thugs and return the money to the shopkeeper
4. Delivery Boy – talk to the Pork Bun Guy
Central:
Drug Busts:
1. Indecent Behaviour / Urine Trouble – Investigate the area and arrest the miscreant.
2. Suspicious vehicle / Bombs Away - another car bomb
3. Public Disturbance / Party Pooper – Investigate the area and arrest the delinquents
4. Suspicious Vehicle / Bomb in the Bay - a third car bomb
5. Plaza Patrol – Investigate the Area – Catch thief
EDIT: 28/04/13: MCoreBE has kindly shared a video link from YouTube (all credit for the video content goes to user XCVii007r1). XCVii007r1 also has many more Sleeping Dogs video guides, so feel free to check out his page if you are stuck.
The following video shows 'Park Patrol', Bomb Be Gone', 'Bomb In the Bay' and 'Party Pooper'.
Now I have written a guide for all the Year of the Snake DLC achievements, I am going to have a pork bun myself XD