After the cutscene, Johnny will call Malc as opposed to requesting help from his club friends. After the call, drive or take a cab to Malc's location in Northwood. Get into his car. At this point you must drive to the "hardware store" where Roman gambles; en route, Johnny and Malc argue about women. Upon arrival, you are instructed to watch for Roman; don't worry if you don't know what he looks like (i.e. you haven't played Grand Theft Auto IV) as a cutscene automatically starts when he shows up, showing Johnny and Malc putting Roman in the back of the car.
Now you need to drive to the warehouse in Bohan and drop Roman off. If you completely stop the car for any reason, such as hitting an object that won't give (like those wooden telephone poles), or stopping to pay the bridge toll, Roman will pop out of the backseat and attempt to run off. Fortunately he runs very slow and falls a lot. Target his head with a weapon to scare him back into the car. Be very careful not to accidentally fire on him. Be prepared to deal with a wanted level if you pull a weapon in view of the cops.
The game provides a much easier solution to the mission, if the player has paid attention to the text messages Johnny receives. At some point in the game, Johnny should receive a text recommending Roman's taxi service and providing a phone number. Once Johnny meets up with Malc, there is no need to drive over to Broker and grab Roman. Simply dial the number of Roman's Taxi Firm (555-2222) and he will come to Johnny and Malc in his car. This makes the drive over to the warehouse easier as it is less likely that Roman will escape from the car, especially if you call the number from near it in Bohan or from Malc's meet-up location north Algonquin, as the toll booth trap does not apply.
A consequence of doing this shorter version is Johnny and Malc do not engage in their conversation about women (unless the player chooses to drive around before summoning Roman - such as getting closer to the drop-off point in Bohan), and there may be less dialogue exchanged with Roman in the car depending on the distance to the drop-off point.