For the unintiated, the Mastermind Rouge lets you use the "Help action" as a bonus action. Not only that, it lets you use it on a friendly ally within 30 feet. 

My question is, how do I describe "helping" someone who is not in my immediate vicinity. Additionally, does the help action really only provide advantage on the next attack roll? Is there another in combat use for it?

The character who is using this is less a master of tactics (contrary to the name of the ability) and more of a smooth talking noble type. She is reasonably intelligent, but she is more Charisma and Wisdom than Intelligence and Dexterity. Basically she is only a rogue because her teacher is a rogue Thief, so they are teaching her the ropes. But she is her own woman, and while she understands the combat utility of Sneak Attack and the like, she is more a "face of the group" type of rouge rather than a "strike from the shadows" type of rogue.

Just distracting an enemy somehow could be justification for giving another person advantage. If you successfully get someone's attention right before your friend swings a sword at them, it's going to be harder for them to block or avoid the sword.

You can flavor it however you want, since I'm guessing the idea of shouting "Hey you!" every time you use help isn't super fun flavor-wise. Per the PHB on help, "You feint, distract the target, or in some other way team up to make your ally's attack more effective." The way you flavor that feint/distraction/team-up might vary depending on the context. If you're right next to the target with your friend, you could pretend you're going to swing at their feet while your friend swings high at their head. If you're far away, you might look like you're about to throw a knife at them or something. Just give it some thought in the moment, and have a few fallbacks (Like the "Hey you!" or "Look, an eagle!") in case you're stumped and feel too much pressure to come up with something cool on the spot.

Execute attack plan alpha! Do the Finagle maneuver! Operation codename porkpie! Darrmok and Jalad at Tanagra!

By yelling out to your team the enemies weakness. "He favors his right leg," "The straps on his armor are loose" or "He cant turn left." Its only marginally insulting insinuating your team doesn't know what they are doing and need you to point out to them how to kill the monster but who is going to say no to advantage? 

Using charisma, you could be distracting with your intimidating or charismatic appearance. As a noble, you could say, "Do you realize who you are facing? Then play the noble card and unload your titles (most of which are probably made up). If you want to go for wisdom, then perception is based around that any way, so just have your character point out a weakness in the enemy's defenses saying, "Strike at the seams under the arms," or something similar. These things don't require an ability roll, obviously, but those are just for flavor using your character's personality.

Help action : "You can lend your aid to another creature in the completion of a task. When you take the Help action, the creature you aid gains advantage on the next ability check it makes to perform the task you are helping with, provided that it makes the check before the start of your next turn. Alternatively, you can aid a friendly creature in attacking a creature within 5 feet of you. You feint, distract the target, or in some other way team up to make your ally's attack more effective. If your ally attacks the target before your next turn, the first attack roll is made with advantage." 

Any ability check would also count so anything that your DM would have you roll a d20 for during combat short of a saving through. For example, grappling/pushing isn't an attack roll its an athletics check. That check would also have advantage. Perception checks to see if more enemies are coming, Investigation checks to find traps/arena hazards to force an enemy into, Insite checks to see through illusions. All of these would be Help action-able. Also, just giving someone advantage as a bonus action every turn on at a range is amazing, so even if that was the only combat utility it gave it would still be good.