Quite easy, but without knowing how would you know… (the docs!)
The Collaborator Quick Start guide is the place to be.
[sudo] gem install heroku
# then
heroku keys:add
#your login details are taken and ssh key uploaded automatically
git clone git@heroku.com:APPNAME.git -o production
#using the proper git repo name.git
if git push heroku fails then do this:
git remote -v
#Hopefully you will see an origin or heroku
#If not, find out your git url:
heroku list
heroku info --app your-app-name
git remote add heroku git-url
(this bit from google groups)