First, navigate to the Discord Developer site at https://discord.com/developers/applications.
From here you will see your applications (bots). If you haven't created a bot yet, see the tutorial above this to learn how. Click the box for the bot you want to create an invite link for. This will take you to that application. Then click to the OAuth tab.
From here you can click the "bot" checkbox, and a link will automatically appear in the bottom textbox, which you can then copy.
Now paste that link into any browser window, and Discord will prompt you to add the bot to any servers that you are an owner of.
Click the Authorize button, and you're done!