First of all, you must know bottle and Google App Engine. Both are awesome tools to any web project. They together make both even easier to use. This integration is easy but isn't too much documented.

Add bottle.py to your source folder and create a helloworld.py file with this content:

from google.appengine.ext.webapp import util
import bottle
from bottle import route

@route(["/", "/:name"])
def hello(name="World"):
    return "Hello %s" % name

def main():
    bottle.debug(True)
    bottle.run(server='gae')

if __name__ == "__main__":
    main()

In your app.yaml, just send everything to your helloworld.py app:

application: sample-app
version: 1
runtime: python
api_version: 1

handlers:
  - url: /.*
  script: helloworld.py

Run dev_appserver.py to test (you already know how to do it if you read Getting Started).

It works nice with last Bottle (v0.9.1). All Google App Engine features are decoupled from webapp, so you can use database, user authentication and others with Bottle.