[Serpentine]

I would suggest using a table; two columns, one for the icon and the other for the description.

First thing's first : Save a copy of your work, be safe.
Add a few icon images to the epub, right click on the "Images" folder and you'll know what to do.

Now on to making the table - at the moment this needs to be done using html however it is fairly easy using the split preview/code view. Swap to that view.

Now, using the code view try to find the position in the text that you wish to place your legend. The <p> tags mean paragraph and you should be able to read them in order. You will want to put the table after a specific paragraph. The closing tag looks like </p>; click into the code, and using enter make a few new lines using the enter key, this white space will not show up in your book, just make it easier to paste. Swap back to the preview area by clicking into it - if there are any errors it will let you know.

Now back in the code view, paste this into the space you made:

<table border="1">
	<tr>
		<td>IconArea</td> <td>Description here</td>
	</tr>
</table>

The table has a single row <tr> with two divisions <td> or cells.

If you click back into the preview panel, you should now see the table in its place.

Simply click into the 'IconArea' cell and insert your icon, then remove the text. The same for description. To add more rows, simply copy and paste a new row following the first; i.e. it would look like:

<table border="1">
	<tr>
		<td>IconArea</td> <td>Description here</td>
	</tr>
	<tr>
		<td>IconArea2</td> <td>Description2 here</td>
	</tr>
</table>

... and so on.

You can also remove the border by removing the 'border="1"'. It just makes it easier to visualize.

Hopefully that helps, otherwise just hop over to :w3schools - tables