I want to know how to do my own Emoji

While you can create small images, you can't create emoji. Because emoji are based on a standard that is accepted by all product designers and manufacturers, so if you send an emoji (which is a binary code) to someone their device must be able to display it. The receiving device will not be able to display your home-grown emoji because its binary code is not included in the standard. The master version of all emoji is maintained by the Unicode Consortium (https://home.unicode.org)

Unless you're talking about Memojis:--> Use Memoji on your iPhone X or iPad Pro - Apple Support

Regards

Giulio