Our friends at WebMedley are sharing a blog series on how to build a template in Arena:

“When beginning your first template, there are a few requirements that we'll need to get out of the way. If you're a developer level Arena client, then the first thing you'll need is the Arena SDK from the Arena Community site.  While this is not a requirement in the sense that you "can't build a template without it", you should use the SDK as it allows you to make mistakes without the whole world seeing it on your production server.  I doubt your tech folks are going to let you go "commando" on a live server anyway, but I'll save you an awkward conversation.  Use the SDK.

If you're not a developer level client, you can still modify the .ascx and .ascx.cs files..."  Read the rest.