Grand Theft Auto IV

To unlock access to the drug wars side missions, you must first complete a few missions from your mothers house. This is located a little ways north of your safe house at the maroon house symbol with the black M in it. The second mission, if I am remembering correctly, has you try to make a drug deal with Armando and Henrigue that goes bad. After this mission, black circle icons that have maroon pills in them should appear on every major island. These are the starting locations of the drug wars side missions.

As far as I can tell, these missions are nearly completely randomly generated. This means trying to tell you exactly what to do would be a futile effort. You'll always get the support of A and H, and they all seem to be fairly short in length, often times just grabbing a bag of drugs and returning, but that doesn't mean it will always be easy. In fact, the game specifically tells you these will get harder over time. This means the enemies will become better equipped and more will show up. Because of this, try to keep a sizable ammo stock and have body armor before taking these on. Call Armando for a gun van to stock up before hand if needed. The completion of each drug war mission will get you a sizable amount of money, so restocking shouldn't be a problem.

So far, I have encounter 4 missions variants, 2 of which have superficial differences and could be considered as the same variant. I'll cover the variants below. I'll add others if I come across any more.

For the first one, I'll cover the 2 variants that are most common and extremely similar, stash stealing and deal break up. In these variants you are given a task to go to the specified location, kill the dealer who's holding the drugs (only in the drug deals, stash missions have the bag on the ground), grab the bag of drugs, and return to your stash. Obviously, there are going to be people in your way. When you arrive at the location, take everyone out that you can see. Once that is done, grab the bag and hop in a vehicle. As you are driving, most likely a few drug dealer vehicles will try to chase you down. I'd recommend just trying to outdrive them and letting A and H worry about them.

Next, we have the vehicle hijacking. In this mission type, the drug dealers are moving a vehicle full of drugs. When this mission starts, a marked vehicle will be driving slowly around the map. When you near the vehicle, it will take off. Don't let them get to far or they will get away. While chasing them down, you will need to kill the driver. To do this, you'll need to shoot him. Try and make sure you shoot the driver and not the vehicle (especially the tires), you'll just make it harder on yourself later. When the driver finally dies, you have to take the vehicle and drive it back to your stash in one piece. Again, you'll be chased by other dealers in other vehicles and again I recommend just focusing on driving.
On very rare occasions, the vehicle will be parked with no one in it. This is a police sting vehicle. When you get in it, you'll instantly get a wanted level. Simply break the wanted level and return to the stash.

The final drug war mission type I have run across is piracy. This is similar to the vehicle hijacking except 1 major difference, it's on water. This makes things substantially harder for a couple of reasons: boats don't provide very much cover for the driver and the waves bouncing the boat make aiming accurately difficult. When this mission starts you'll have a specified boat at a dock marked. This is the boat you'll need to use to chase down the drug dealers. From here, things are fairly similar. When you get close, they take off. Try to kill the driver with minimal damage to the boat. You'll then have to get the enemy boat (your character can pull himself out of the water by hitting Y near a boat). Then take said boat to where you are told.
Just like the vehicle jacking, the boat you are told to enter at the beginning may be a police sting. Just break the wanted level and return to the point.

Mister Buds reports there being another mission type I never encountered myself. I'm not surprised though as many of the additions I made to this guide were when I had less than 5 to go. Mister Buds said:A few more notes here. If you get noticed by the cops at any point, you'll have to break your police notoriety before returning to your stash. After every drug war mission you complete, a tally screen will pop up notifying you of your progress towards this achievement. And finally, after completing 25, they will still appear on the map. You can continue doing them if you need cash.

As a final note, Mr Andy Dixon added a reason you may wish to continue the drug wars missions beyond the 25 necessary for this achievement (according to AbmaOnline, you'll also have to be past the point in the story where said weapon appears). Mr Andy Dixon said: