These are my notes I've collected on FF6MDE (Final Fantasy 6 Miscellaneous Data Editor). Please know that I am not 100% sure on everything, although I'm "fairly" sure about these; I have yet to master the program. What I'm covering is mainly about Espers and how you could make your own based on my findings.
The objective we're going to try and complete is a custom Esper called Asura, so the first step would be to open four copies of FF6MDE.
The first copy is your official version, the one you're editing.
The second will be to give the official copy the bytes of your Esper.
The third copy is of the mold of another esper. (If your sprite is to large for an existing summon, you can use an existing summons byte-mold).
the fourth copy is for any custom spell animations it'll be casting.

What I'm wanting Asura to do is summon next to my people and cast a Cure spell using Cure 2's animation. The objection is to make it look as close to FFIV's version as possible while staying within FF6's graphical limitations.

Animation 1:
Byte 1 - Mold(Side Animation Size. Ie, Golem's falling rocks or steam)
Byte 2 - Mold (In almost all cases, you cannot change this from what is already existing. It coexists with Byte 1. Changing this would most likely glitch out your creation.)
Palette - Color of the animation.

Animation 2:
Byte 1 - Mold(Main sprite animation size/movement animation. Ie, Bahamuts size, but his introduction animations.)
Byte 2 - Mold
Palette - Color of the animation.

Special Animation:
Byte 1 -
Byte 2 -
- Spell Animations / Summons Graphics

If you're creating a custom Esper that's overwriting a smaller Espers sprite, chances are that when you test it, part of your sprite will be either cut off or distorted some fashion. As I said, you can however use an existing Espers mold which is what I did. The third window towards the bottom (Bahamut) is what I used as a bases for Asura's new mold because her sprite is much to large for Siren. Bahamuts "Animation 2: Byte 1" is Bahamuts mold, but also what he does at the introduction. Bahamut when summoned is first in the middle of the screen, then moves towards the player. That's his introduction.
To get her to cast Cure 2, just copy over her bytes.

It may seem simple from the explanation although if you get one byte wrong, it's often catastrophic. Most of this is trail and error since the editor didn't come with a tutorial or was labeled well at all.
I have come across times when certain Espers molds are in Animation 1, Byte 1 and 2 instead of Animation 2, Byte 1 and 2.
This is a bit incomplete, although these are my notes on the daunting subject of custom spells and Espers in FF6.
The end result, Asura.