First of all thank you very much for your interest in creating a new appliance!
These appliances are normally created by OpenNebula team. But, in the case you want to contribute, you can find all the information here. The only thing is that the image(s) that you want to publish, need to be send to one of our members (for example me) in order to test it and publish it in the public marketplace.
There is no template to build that. The files that need to be created and all the information depend on the appliance you are building. The only guidelines are the ones specified in the marketplace repository.