I have no clue how to do this.

Like a swarm of insects or a horde of weak creatures?

Quote from Samso1 >>

Like a swarm of insects or a horde of weak creatures?

 same question. could you please elaborate more? I'm happy to help, but you did not give me enough information to help you. 

A swarm is essentially just a group of creatures that, in combat, functions as a single medium or larger creature. As the characters damage it, the number of tiny creatures gets reduced... think of the HP as literally being the number of creatures still in the swarm. They're still functionally a single creature for the purposes of spells that target a single creature, but most swarms are immune to a lot of conditions, mostly to represent the fact that certain common effects couldn't apply to a swarm. Like... you can't knock a swarm of rats "prone", and you can't really charm the swarm, since most charm spells target a single creature, and charming a single rat in a swarm wouldn't really have any effect on the swarm as a whole.

Actually just last week I ran of swarm of undead snakes in Frozen Sick.  So they're a medium sized monster from the start with the ability to occupy the same space as an individual.  Once reduced to half hit points the damage they deal out is also half.  Fluff wise I narrated attacks depicting the particular round's victim shaking multiple biters off of limbs and noses.  One character tried to use a shield as an improvised weapon to sort of drop slam the swarm when their space was no longer co-occupied (having been cunning action disengaged by a rogue).  It broke some of the action economy, I was only going to allow improvised weapon damage anyway, but the attack roll failed so I described the bones slither scattering away around and over the shield sort of like you'd see with silverfish or roaches.  

I'm not being snarky, but you see the picture?  Just run them as a stat block and like TransmorpherDDS points out, govern their actions and targetability with common sense (no one tried any missile attacks, and if they were a flying swarm, I'd really have to think over whether I'd allow anything to happen or just treat it as a wasted action learning point for the PC).

Oh, one character got a bit ingenious, recognizing after a couple of rounds that they were resistant to conventional slashing damage, a war magic wizard with martial proficiencies asked if they use green flame blade, strike the ground adjacent to the swarm and let the flame leap onto them.  I allowed it, and maybe buffed it a bit because I thought it was a good idea in the mid of a party of 2nd level characters really freaking out over this (whereas zombies, a Wight, giant crabs and ice mephitis totally unphased them) and I wanted to wind things up. 

Quote from DataDwarf >>

You can use the Handling Mobs rule from the DMG

 That's a great tactic for a larger number of any combatants with individual stat blocks and you want to avoid rolling like twenty attacks.  A swarm as stetted in the monster manual doesn't work like that (though similar principle in the assumption that the attacks and damage dealt are collective efforts of the swarm, it's just consolidated in one stat block).

Thank you I read through it. I'm home brewing a swarm of bunnies. They used to be rabbit folk but lost their mind and became consumed with only one desire; to feed. I've never homebrew a monster before and I don't really know how CR works or hp. Could you guys give me some pointers? I know this creature is rough lol

https://www.dndbeyond.com/monsters/1584749-the-great-rabbit

I think you need to think through at minimum the action economy.  Read more monsters and see what Legendary actions actually are (I think you're confusing them with attack actions) and move the bonus actions into legendary actions and determine how many legendary actions it will get per turn.  

How does swarm of rabbits dismember.  I mean giant swarm of rabbits is for laughs, but it's better to be ridiculous in terms plausible to rabbits or rabbit "lore" (so alertness, vision, quickness, hopping etc) than go ridiculous when your monstrosity is no longer mapped to any sensible understanding of the "base beast so to speak."

I think you've also written yourself out of the swarms ability to move through ratholes, the whole gargantuan swarm can move into that?  

I mean the concept could be fun, but I frankly think there's just too much going on in it.  Swarms are simple, even a legendary one.  Rather than writing it's tactical options of "I shall be..." just cut to the quick, so to speak, and describe the action.  

If you've never homebrewed a monster before, I'd recommend starting at a lower level. Higher level stuff is complicated and requires a fine touch to hit the zone between a pushover and a TPK.

Funny concept though!

Wow I really appreciate your feedback

I like the I shall be thing cause it fits with another lore part. Although yeah I don't really know how legendary or mystic actions work at all

So, respectfully, the only way you're going to homebrew a monster that works, is by understanding how monsters work.  If you don't know what legendary actions are, you ought to read more monsters that have legendary actions to see how they actually work.  Your presentation just looks like you have a rough idea of what a stat block is but you don't seem to have a sense of how a stat block functions.  Read more about the monsters and actually use the monsters in game to get a sense of how the game actually works before creating something new for it.  You got to know how to drive and the basic rules of the road before you start hotrodding.  Otherwise your game just crashes or stalls.  

Let me try and talk about Legendary Actions and Legendary Resistances... first of all, you've probably heard the term "Action Economy", which basically means that the more Actions one side of combat are able to take, the more they're likely to succeed in combat. If, instead of treating all your bunnies as a Swarm you treated them as 1,000 individual bunnies that each get a single attack per round, even if they only deal 1 damage each and have a low chance to hit, they'll still outclass your PCs just by the sheer number of them. Also combat will take forever and be really, really boring, which is a good reason to treat a huge wave of bunnies like a Swarm instead of individual creatures.

Anyway, big, dramatic boss fights with PCs versus monsters usually takes the form of a single large, dangerous enemy versus a group of 4 or more characters. The Big monster gets a single turn, so even though it's got more health and a stronger attack, it's still likely to get swarmed by the PCs. Someone hits it with Hold Monster or something right off the bat so then it just gets pummeled by everyone else unloading their strongest attacks and spells all at once. That's where Legendary Actions and Legendary Resistances come in.

Legendary Resistance is much easier to explain... if the creature fails on a Saving Throw for any reason, a Legendary creature is powerful enough to just force itself to succeed anyway. This means if someone successfully hits it with a debilitating spell like Hold Monster it can just force itself to succeed so that your players can't just stun or polymorph or whatever against their enemy and get a free uninterrupted round of combat. Legendary Monsters get a set number of Legendary Resistances per day, so if the players have multiple encounters against a single Legendary Monster (like if they or it run away from combat) it can't just keep resisting spells and abilities all day long.

Legendary Actions are unique actions meant to let a single monster make strategic actions outside of its turn. These aren't the same as reactions, but after another creatures turn a Legendary Monster can use one of its Legendary Actions. It can do this after any other creature's turn... usually after a PC's turn, but if the Legendary Monster has allies, it can take Legendary Actions after their turn as well. This is meant to make the Legendary Monster kind of function as multiple creatures to allow it to keep up in the Action Economy. Each Legendary Monster has limited number of Legendary Actions, but it regains all of its Legendary Actions once per round, so don't forget to use your Legendary Actions... there's no benefit to trying to save them for later.

Interesting, I was running mini bosses before and I ran into the action economy problem but that makes sense to use something like that. I will move things around and see how it works