Two methods must defintely be redefined RENDER and RENDER_DATA. In the Render method you can write the HTML which should be displayed.
To make the Item known by the WAD you need to create an entry in Table RSRRENDERER.
Most of the Fields are self-explaining. with Data_bind indicates whether a Item must have a Dataprovider assigned or not. You can find the standard images in the temp folder of your harddisk under %temp%\bw if you want to use own images you must upload them to mime repository under:
Now you need to add the attributes of the Item to Table RSRRENDERERATR. The Attribute NAME is mandatory and must contain the Class name as value.
If you now start the Web application designer, the new item should be visible in the Item list. Now you can drap and drop it into you template. If you save and execute the template you item should be rendered in the html.
But please be careful if you use own items in Templates SAP will not make any support for this template also if the error comes from a standard item. You will be forced to remove own items, to get support.