---Developer site---
So, first you need to register as a developer. This takes just a minute. Once done, you need to start building your layer definition. This sets things like the URL of your webservice, and the name of the layer. You can also use the developer area to test your layers before submitting them for publication. Again, documentation is provided. But I found it all pretty straightforward and easy to work out anyway (they could do with some better web designers though!)
via Matthew Bull
http://www.matthewbull.net/2010/07/11/building-augmented-reality-layars/
First Layar Tutorial - Create a simple layer
http://layar.pbworks.com/First+Layar+Tutorial+-+Create+a+simple+layer