After cloning the repository you'll need to install its dependencies via bower. And this component is a bower package itself its .bowerrc file sets the bower directory to ../. To keep your file structure neat you can create a dedicated component folder where you can clone this repository and install its dependencies.
$ mkdir polymer-components
$ cd polymer-components
$ git clone git@github.com:prtksxna/leaflet-map-component.git
$ cd leaflet-map-component
$ bower install
This should set up local development. You can now go back to the polymer-components directory, run an HTTP server and navigate to localhost/leaflet-map-component/. If you have Python installed the simpleHTTPServer should do.
$ cd ..
$ pwd
~/code/polymer-components
$ python -m simpleHTTPServer
Make sure to update documentation if you change an existing method. Use JSDoc strings to add documentation.
Create a file in the root of the project, for example leaflet-rectangle-component.html. You could also copy an existing file to get boilerplate code. If the element is going to be a child element of <leaflet-map> then you'll have to add it to the select attribute of the <content> tag in leaflet-map-component.html. After writing the new component import it at the top of the main leaflet-map-component.html file like so:
<link rel="import" href="leaflet-rectangle-component.html">
Also add it to the sources attribute of core-component-page tag in index.html so that its documentation is generated. You must also make sure that it behaves well with the fitToMarkersChanges method of <leaflet-map>.
If you've updated demo.html or demo_files/ you'll have to regenerate the Github page for the repository. This is done using a flavor of Polymer/tools. Clone this fork of the Polymer tools in the polymer-components directory you might have made earlier and checkout the leaflet-map-component branch.
$ cd ..
$ pwd
~/code/polymer-components
$ git clone git@github.com:prtksxna/tools.git
$ cd tools
$ git checkout origin/leaflet-map-component
Now, create a temp directory inside polymer-components/ and run tools/bin/gp.sh, passing it the repository's owner and component's name.
$ cd ..
$ mkdir temp
$ cd temp
$ ../tools/bin/gp.sh prtksxna leaflet-map-component
This will push an updated copy to the gh-pages branch.