This guide takes inspiration from Photon's "How To Make Your Own" series. In case it's not obvious, this guide is mostly a joke.

Let's get straight to the point. To build Supreme Calamitas, you'll need:
10 Shadowspec Bars
All of human Calamitas' combat drops (except for the Malice drop)
1 large Brimstone Jewel
The Grand Design (to help make the thing)
A music player (set to Stained, Brutal Calamity or Cinis Cinerem)
Eye of Extinction (to respawn your project if something goes wrong)
Arena Block Generator
Altar of the Accursed and Ceremonial Urn (for testing the prototype)
A video recorder, for analyzing fights
A place to build without attracting suspicion
Good engineering skills
Good thieving skills

Can't find Shadowspec Bars, a music box, a Ceremonial Urn, an Eye of Extinction, an Arena Block Generator, an Altar of the Accursed, a video recorder, and Calamitas' weapons? No worries! Just find Calamitas' super secret hideout containing all of these things! Then, wait for someone to waste her time, and quickly go and steal all her stuff! Run out of space in your inventory? Money Trough exists for a reason!

Forge the Shadowspec Bars into SCal's old form from Calamity 1.4.
Wire up the weaponry, music box, and Eye of Extinction using magic and the Grand Design.
Insert the Brimstone Jewel as shown in this guide's picture. It will be the source of all the attacks that don't come from Calamitas' weapons, so be careful with this step.
Insert in the Arena Block Generator. It can be used to place and break Arena Blocks in case a fight breaks out.

After building it, go find Calamitas herself. You can do this easily with an Altar of the Accursed and a Ceremonial Urn, both stolen from Calamitas. Then, attack her. You can respawn, so if you lose, simply use the Eye of Extinction and adjust your design.
Repeat until you win the fight. This means you have succeeded. Now, the question is what to do with the now freed Calamitas... This is a story for another time. I suggest killing her.

Well, it's up to you. You should probably rescue Silva from the Abyss by taking inspiration from Calamitas and burning down the Abyss. Be warned that Silva might want revenge on the very person who almost killed her, so stay out the way of the ensuing battle between Silva and Calamitas. You should record it too.

DO NOT CHALLENGE YHARIM. Also, DO NOT FIGHT THE EXO MECHS WITH THIS. MrJFL has a cheese strategy for you, over here.