So, I am really struggling with creating my homebrew.  It seems like I am doing everything right, but obviously I am missing something here.

Example: I made a homebrew that was intended to give a character an equivalent to Dragonborn  breath. It is a mess. First of all, there is no entry for snippet code! All I want is to display the DC of the saving throw!  I thought that if I just put in {{savedc:con}} in the text, it should display the saving throw based on proficiency plus the users con bonus. But no dice. It just shows the code as text. I also tried {{modifier:str}+{proficiency}+8} which should be the same thing.

Ideally, this should just be a magic weapon, so it would show as an attack, but I can't make that work either. In a truly perfect system, the user would see the flame breath as an attack with damage that scaled with level (as on the standard breath weapon) and the item would show a single charge.

any help is appreciated.

For the feat, you will have to add an Action and then you will be able to display it exactly as you want.

The snippet code you will want to use is {{savedc:___}} and put whichever ability score you are using whet the underscore is.

how is that different than what was included in my question? I put  {{savedc:con}} in the text of the description of the item. However, it just displays as that text and doesn't operate as a snippet.

Snippets don’t function when entered in the “description” field, they only work when entered into a “snippet” field. I read that you did not find a “snippet” field. Did you actually add an action to the feat to allow the character to actually use the feat? I’m willing to bet you did not, or if you did, you didn’t scroll down all the way because every action has a snippet field.

I created it as a weapon, not a feat.  It sounds like I need to re-create it as a feat? The problem with that is it means it will show up as a feat available to all my players. Are you saying I cannot add an ability score based DC to a magic item?

My apologies. It never even occurred to me that you would have made it as a weapon. Weapons use “attack rolls,” they don’t force saves, so there is really no need for a save to be associated with a weapon, unless the weapon grants the ability to cast a spell that requires a save. You could attach Dragon’s Breath to the weapon, or if that doesn’t suits your needs you would have to homebrew a spell and attach that. Keep in mind that a magic weapon will always have the same attack options that the base weapon has, so unless you want this thing to be a sword or whatever all the time too.... Of course,, if the weapon only does this, you could make it a Wondrous Item and not have to worry about it. However, if you make it a feat, you can attach an action, set it to “spell action” and have it do exactly what you want all in one homebrew. 

Quote from SethTheFrank >>

I created it as a weapon, not a feat.  It sounds like I need to re-create it as a feat? The problem with that is it means it will show up as a feat available to all my players. Are you saying I cannot add an ability score based DC to a magic item?

 Also, if you make it a Magic Item, all of your players would still be able to find it if the seats on their character sheets for items. The way around it is to create “it” (weapon, wondrous item, feat, spell, race, subclass, etc.) attach it to the character it is intended for, and then simply “remove [it] from collection.” It will stay in “your creations” and on the character it was assigned to, and everything will function as it was programmed, but nobody besides it’s intended recipient will be able to find it unless you publish it.

How would you apply a snippet to a Wonderous Item?  They don't have an action option either, or is this not possible.

Snippets work in any field named “snippets,” if there is no snippets field, you cannot add a snippet. 

It does not matter what kind of magic item, currently we cannot add actions to them.