As number one is a bit vague, and having a list with only one item would be pointless, the rest of the items on the list will let you know what everything means.
2: Appearance: It may seem pointless at first, but giving a detailed description of your custom's appearance and the appearance of your custom's techniques (when applicable) allows the person that's going to go over it with you the ability to visualize what you're going for, being able to do this reduces confusion by a great deal.
3: Everything that deals or mitigates damage: The reason for this should be obvious, but for the sake of completion I'll explain anyway. If there is something that you do that deals some obscene amount of damage while you're doing it, but you describe it as if it's just a normal attack or ability then it's going to be treated like a normal attack or ability that shouldn't cause that much harm and it's going to be called godmodding when you use it in game and it causes that obscene amount of damage. The same applies to something that is capable of reducing or negating some form of damage; if you say your release gives you a shield of some sort without elaborating as to what that shield can do and how much damage it can take, then it's going to be taken as a normal breakable shield and people will expect that they'll be able to break it without much consequence. You don't want that do you?
Addendum: Thanks to the new custom system damage is now capable of being displayed as a modifier to a technique, this is true for custom abilities/attacks as well. As such when describing your attack or ability you must now decide on a modifier that properly reflects the damage your attack gives. As a note; no modifier above 15 will be accepted. 15 is the base modifier of a black cero, one of the strongest offensive abilities in-game, and as such it will be nearly impossible for any one custom technique to have a modifier higher than this. The possibility is not non-existant, but in making an attack/ability with this level of power it would be wise to keep in mind that it will more likely than not be denied. To give a more concrete baseline as to what abilities can do; Haien has a modifier of +6 while, as stated, Black Cero gives +15.
4: Everything that causes a change in yourself, your opponent, or the environment: If you have a release that causes changes just because it exists then these changes need to be known and noted. For example, don't assume that just because you say your release controls fire that it will be immediately taken into account how large of a temperature change in the surrounding area that would cause. Yes; your fire is hot, but how hot is it? This applies to other things as well, if you have a release that causes disentegration then the area of effect needs to be noted and the rate of disentegration needs to be written down as well, it needs to be known what happens to the world around you when you fight, and as is said by the title of this item, note the changes in yourself and your opponent as well.
5: Attacks: If you've got a special kind of attack that belongs to your release or ability then we want to know about it. Everything about it, this includes but is not limited to it's size, speed, AoE, the collateral caused by it, how long it takes to charge, how long it takes to fire, how much energy it costs, how much damage it can cause to an enemy, and how much damage it causes to it's user when used. Canon examples can be used for these things.
6: Abilities:: Obviously if you have a custom it has abilities, and obviously we want to know about them. The same information that is needed for attacks is needed here.
An extra guide to costs and what can be done using the custom system. http://pastebin.com/pB6nWrCW
1: Energy Costs: As this seems to be skipped a great deal, it's now going to be the first on the list. Whether it's an ability, an attack, or a transformation of some kind, everything has an energy cost. It is understandable that some things are of so little note that they don't need to be given one; likely because the energy that's drained is so infinitesimal that it's inconsequential to a battle. In this case it is understandable that you don't place a cost, if it is an attack though then it must have an energy cost, and it must be reasonable. I'll tell you now that there is rarely ever a time in which you'll have something that only takes 1% energy if it's an actual attack.
Addendum: Thanks to the new custom system energy costs can actually be implemented into techniques. As such when describing your attack or ability you must now put an energy cost (in the form of a whole number (I.E. 5, 1, 3, etc)) instead of a percentage. Note that as energy costs cannot be changed as you master an ability ic'ly the cost that you choose upon the creation of your custom is the cost that will remain throughout your use of it.
2: Don't give yourself a boost greater than your native release: This means that you'll never be able to recieve more power than your native release gives; I.E. you can't give yourself extra strength, reiryoku, or (and there's really no reason you should even try this) health. The reason for this is that it can't be calculated properly and it's highly unfair to those people that don't use customs and rely solely on the power of their native release. This isn't to say that you can't find a way to make yourself stronger, but you have to do so in a way that is approvable.
3: Don't ask for something unreasonable: I'm not explaining this one because I really shouldn't need to. If you take this part as a joke then we'll take your entire custom as one too.
4: Don't give yourself more abilities than a Swiss army knife has tools: While there likely won't be an exact limit placed on the number of abilities you're actually allowed to have I will make it known that if you have an excessive amount you're more likely to have your custom rejected entirely or to have a number of your abilities stripped.
5: Don't screw around with time: You can't control time. This shouldn't need an explanation either, if you do it then I'll approve your custom and immediately force your characters powers to malfunction horribly.
6:Stat Type:: Custom techniques are now all connected to a single one of your four stats (Physical Strength, Reiryoku Strength, Physical Defense, and Reiryoku Defense). As such when describing an ability you will also need to note exactly what stat your ability runs on for damage or damage reduction. Simply state one of the four stats and your custom technique will be rolled based on it when it is given to you.
BYOND key: (Self-explanantory)
Character name: (Self-explanatory)
Race: (Hollow, Human, Quincy or Shinigami.)
Release: (This is what you got for your rolled Shikai, your Fullbring type or your Ressureccion. Quincy and normal hollow can leave this blank.)
Appearance: (What your custom looks like either in its released state, in general, or (more appropriately) in all forms. This is to give an easier time in visualizing your abilities, which makes approval a great deal easier.)
Selected Element(s): (This is where you put the selected element for your release. Please be sure to do so in the order that the elements were selected.)
Custom Attack #1
Name of Ability: (Self-explanatory)
Attack type: (ex. Simple Projectile, Homing projectile (and the amount of + you want), piercing (and the level you want), etc.)
Damage type: (ex. Physical, Reiryoku or Defense.)
Modifiers: (ex. damage, multi-target, large range, etc.)
Status effects: (Place applicable status effects for your elements here.)
Description: (Describe the manner in which your ability is performed. Note that once this is decided the attack must work in the manner described. Its manner of execution may shift slightly nothing else.)
Custom Attack #x
When you have finished designing your custom and placing it in the proper format then you are to place it in pastebin, afterwhich you should give the link to your custom directly to an admin. Be sure to set your custom to a 30 day expiration date as well as unlisted.
These rules are subject to change whenever a new rule is thought up, as this will likely not happen often you will probably be informed of changes. Probably meaning that you should still check in from time to time.