Epic Boons application

How would I apply epic boons to a new monster stat block, also their isn't an Epic Boon option for monster stats in the custom monster maker

That's because these are meant for players...

Quote from Felgrand4123 >>

How would I apply epic boons to a new monster stat block, also their isn't an Epic Boon option for monster stats in the custom monster maker

 You'd give them a power replicating what the epic boon does. Which is the way all feats work.

So would I make it like a Legendary Action?

Quote from Felgrand4123 >>

So would I make it like a Legendary Action?

That would depend on which Epic Boon you're talking about and what kind of effect it has. Some give you abilities you can use as an action, others are Reactions or passive effects. It's up to you how your own homebrew monsters' abilities work.

Let's say an Ancient Red Dragon with the Epic Boon of Terror, hows that

Quote from Felgrand4123 >>

Let's say an Ancient Red Dragon with the Epic Boon of Terror, hows that

• In condition immunities, give it immunity to Frightened
• In skills, give it expertise (+22) in Intimidation
• In reactions, give it Flee, Fools.

Note that, since an ancient red dragon has no inherent means of generated a frightened condition, and it can just cast command to make fools flee, this won't do much.