Objective

Make a plugin that replaces the Deeeep.io logo with <Your Name>.io

Prerequisites

A basic knowledge of JavaScript is required. If you don't know it, JavaScript.info is a good place to start.
You will also need the Plugin Dev (plugin-dev) plugin by Canned Seagull (DRC's dev) installed.
The plugin is not required for building plugins but will make your life really easier.

Setting Up

Open the Plugins window. With the Plugin Dev plugin installed, click on the blue "Developer" button, then click on "new".
You should now see a plugin editor appear.
Fill in your plugin's name, description, and your author name. Under "ID", fill in any unique identifier name containing only alphanumeric characters, the dot, underscore and hyphen.
Example:
Name: My Plugin
ID: my-plugin
Description: A testing plugin
Author: Me

Writing the Code

Click on "New Script". You should see a new script section appear.
Under the left dropdown, you can select a script type. Select "domloaded". This will run the script on DOM load.
Under the center text input of the script, enter the following code, replacing "<Your Name>" with your name.
const logoWrapper = document.querySelector("div.el-image");
logoWrapper.innerHTML = "";
const text = document.createElement("h1");
text.setAttribute("style", "font-size:3em;height:1em;white-space:nowrap;line-height:1;");
text.innerText = "<Your Name>" + ".io";
logoWrapper.appendChild(text);

Completing

You created your own Plugin! Restart Deeeep.io Reef Client to see the plugin in action.