Players (and monsters) benefit from the Optional Flanking Rules as described in the Dungeon Master’s Guide 5e.
When a creature and at least one of its allies are adjacent to an enemy and on opposite sides or corners of the enemy’s space, they flank that enemy, and each of them has advantage on melee attack rolls against that enemy.
When in doubt about whether two creatures flank an enemy on a grid, trace an imaginary line between the centers of the creatures’ spaces. If the line passes through opposite sides or corners of the enemy’s space, the enemy is flanked.
A creature can’t flank an enemy that it can’t see. A creature also can’t flank while it is incapacitated. A Large or larger creature is flanking as long as at least one square or hex of its space qualifies for flanking.
Healer’s Kit Dependency
A character can’t spend any Hit Dice after finishing a short rest until someone expends one use of a healer’s kit to bandage and treat the character’s wounds.
Medicine Skill and Healer’s Kits
In addition to the current rules regarding the Medicine skill and Healer’s Kits, Players can use the Medicine skill to diagnose and treat certain conditions the target character may be currently affected by, and grant the target character an additional saving throw against that condition.
On a successful Wisdom (Medicine) skill check, a player can grant another character a fresh save against Poison or Disease at the same DC as the initial saving throw.
If the player making the Wisdom (Medicine) skill check has a Healer’s Kit, they can expend one usage to grant the target character advantage on their new saving throw.
The player must declare whether or not they are expending a usage from the Healer’s Kit before they make their skill check.
This skill may be used to treat other negative conditions (such as Blinded, Deafened, etc.), under certain circumstances at the DM’s discretion.
If the target character fails their new save, their condition remains unchanged.
Flanking illustration by Unknown, courtesy of Reddit