This will enable some extra options and also a new tab in Admin CP called Development.
Click that and then click on "Create Add-on".
Here fill in the details. Generally the ID should be the add-on title in CamelCase so if your add-on is called "My Add-On" then you should set your add-on ID as "MyAddOn". You can actually make it whatever you like but it should be unique.
None of the other options really matter.
Now your add-on is created, you can create your Template Modifications.
Always ensure that you create your Modifications in the "Master Style" which will have become available since Debug Mode was on. Also, while you're creating template modifications to the Master Style you should now notice a dropdown box at the bottom of the page that allows you to specify an add-on.
You should set the add-on for each Template Modification as your own.
This saves all of the modifications against your add-on.
Finally, go to your list of add-ons and hover over the "Controls" menu next to your add-on. Click the option "Export" and an XML file will download.
That XML file contains all of the template modifications.
Add-on complete. Zip it up and add it to the Resource Manager.