How do I create Genmojis and use like an emoji on my Apple device?

I love being able to create new emojis, but they can’t be used like regular emojis. They are giant and replace your message instead of inserting into the text. And they fool you by appearing in your emoji library as if they are a regular emojis. How do I create emojis I can use like an emoji?

[Re-Titled by Moderator]

Original Title: Genmojis too big!