AllTheMons x Mega Showdown

Singleplayer:

Put the .zip in /saves/[WORLDNAME]/datapacks
Put the .zip in /resourcepacks
Enable in game ESC - Options - Resource Packs

Servers:

Put the .zip in /world/datapacks
Sometimes it's called myworld, or something similar, sometimes it has a name instead
If you're in doubt about the correct folder, consult your server providers guide on how to add datapacks

Every Player has to have the .zip installed as a Resource Pack
Best option if for them to have it installed and enabled in their clients
It's possible to add them to server resources, but note that there have been issues with this for some cobblemon addons, so try whatever method suits you better

If you want to know how each of those forms look like, consider checking the gallery.
As mentioned above - those will not show in creative, JEI/REI/EMI, you have to craft them. Also note that all of these have been removed in v3.3.0 and moved to Lost Lore

Ancient Light Ball

• Give Pichu to hold (cosmetic slot) to turn it into the HG/SS Spiky Eared variant

Binding Armor

• Right-click Mewtwo to turn it into his Armored variant

Corrupted Wish

• Right-click Groudon to turn it into his Meta form (Wish Maker Movie)

Green/Red Scarf

• Give Lucario/Riolu to hold (cosmetic slot) for a Mystery Dungeon inspired scarf

Pledge Fragment

• Give Keldeo to hold (cosmetic slot) to turn it into its Resolute form (cosmetic only)

Shadow Energy

• Right-click Mewtwo/Lugia to turn them into their Shadow variants

Time Gear

• Give Dialga to hold to turn it into its Primal State (from Mystery Dungeon)

Water Shuriken

• Give Greninja to hold (cosmetic slot) to turn him into Ash Greninja (cosmetic only)

Shadow Mewtwonite X/Y

• Give Shadow Mewtwo to hold so it can Mega evolve into the Shadow Megas

Slowbrogonite

• Give galarian Slowbro to hold so it can Mega evolve into a Galarian Mega

Costume Boxes

• Right-click Lucario/Incineroar with their respective box to switch costumes

Due to the collaborative nature of this addon and the amount of different assets added - there's lots of incompatible mods and addons. The lists below include stuff that is incompatible with Mega Showdown itself (since well, this addon requires it)

Premade Modpacks:

• You can get it to work but since they might use lots of incompatible addons for core aspects of gameplay, it’d be a lot of file management to fix it. You're better off leaving it as it is since or make your own one with the entire incompatible addons list in mind. If you want to make it work any modpack, your best advice is asking the modpack author or people in their discord for help/advice.

Incompatible Mods:

• Ascension Megamons, Cobblemon Alphas, Cobblemon Megas, Cobblemon Showdown Assets, Cobblemon Drop Loot Tables, Cobblemon Updated Enhanced, Complete Cobblemon Collection (CCC), Flourish, Genesis Forms, Gimme That Gimmick, Gravels Extended Battle, Hiddenmons, Hulis Z-Crystals, Legends Untold (Reborn), Myths & Legends

All addons that contain any overlapping Pokémon/Files, here's some examples:

• AllTheMegas, CardboardMons, CavsCobbleMons, Cobblemon More Level Up Moves, Cobblemon Radical Changes, CobbleRemodels, Complete Cobblemon Collection (CCC), Coolrex's Cobblemon Collection, DracoMon, GlitchDex, Journey Mounts, Legends & Myths, MelloMons, Missing Legends, Mysticmons, Myths & Legends, Planeta's Pokémon Pack, Pokeodyssey, Shadow Lugia Addon, Soul National Dex Learnsets, Soul Megas...

All addons that are already included in AllTheMons x Mega Showdown:

• Allones Additions, Ashimons, BoniMons, Cloudmons, Cobblecats, Genomons, HiddenMons, Hydro Reanimodel Pack, Kale's Collection, LackingMons, MissingMons, MonkeyMons, MoreMons, MundialMons, NadyMons3, OdysseyMons, Pigeons PokePack, PokeCostumes, Pokemans, TDmons, The Jewel Pokemon & WaviMons

All other versions of AllTheMons:

• This includes the regular ATM addon, other forks like BCG x ATM and all older versions of ATM x MSD

All Developers of these packs gave their full permission for their assets to be used, proof of these can be found within the addon file, in a folder titled "Proofs of Permission"
We did not create any models, textures, or animations, that's not to say the ATM team didn't do anything though - this pack has hours and hours of work put into its spawns, species files, bug fixing, creating documentation, and much, much more, all done by EasySqueeze, Mayayoh, N8nt and myself.
Due to the collaborative effort, AllTheMons has a strict license to make this even possible in the first place. Read it before starting a server or modpack containing the addon. You can find it here.