If you were asking how to install tMod then go to their thread, not here. The installation guide for the Calamity Mod at the top of the original post is incredibly simple to understand.
To everyone complaining about the despawns I have since changed them for the upcoming version. Just waiting on the new statigel armor to be sheeted before I release it. I have listened to the comments and I've changed how the despawns work. As in, most bosses only leave if you exit their biomes. Here is how they all work:
Desert Scourge: Will burrow into the ground if the player leaves the desert biome for too long or if all players are dead.
Hive Mind: Will fly into the ground if the player leaves the corruption for too long or if all players are dead.
Perforators: Will burrow into the ground if the player leaves the crimson for too long or if all players are dead.
Slime God: All parts will despawn almost immediately by fading away and falling into the ground if all players are dead.
Cryogen: Will fly into the ground if the player leaves the snow for too long or if all players are dead.
Brimstone Elemental: Will fly away if the player leaves the Underworld or if all players are dead.
Calamitas: Will fly away if all players are dead or if the sun rises.
Leviathan: Will fly into the ground if the player leaves the ocean biome for too long or if all players are dead.
Plaguebringer: Same as before. Will despawn and bomb the player if they leave the Jungle for too long.
Guardians: Will fly away if all players are dead.
Providence: Will fly away if all players are dead.
Ceaseless Void: Will despawn if the target is too far away for too long.
Storm Weaver: Will fly into the ground if all players are dead.
Signus: Will fly away if the player leaves the Underworld or if all players are dead.
Devourer of Gods: Will burrow into the ground if all players are dead or if the sun rises.
Bumblebirb: Will fly away if all players are dead. Despawns very quickly if the player is not in the Jungle and gets too far away.
Yharon: Will fly away if all players are dead.
Supreme Calamitas: Will fly away if all players are dead.
So please, save your criticisms for the next version or they will likely be ignored.
Respond with your own critiques to these plans, but they're likely not going to change at this point.
Keep in mind that this mod is NOT designed around ANY other mods out there. I don't care if you're using some stupid "instant respawn" mod, or anything like that. What conflicts occur between this mod and others is none of my concern.