Let's start with a very simple, trivial example that will help to explain what's involved in creating Custom Tags.
The first step is to decide what the end product of our Custom Tag is going to be. In this simple example, we're going to create a heading tag and underneath it a div that contains a message. An important part of this step is to determine what parts of the end product are constant and which parts we want to be variable and controlled by the user. In this simple example we'll want the message to be variable. I always recommend you write out an example or instance of the structure you're wanting to create as the end product, eg:
<h4>Message for Today</h4> <div class="messagePanel"> Hello World! </div>
In this simple example, the text "Hello World!" is the variable part. Everything else will be fixed.