Generally, you will add a git remote for your Heroku app during the Heroku app creation process, i.e. heroku create. However, if you are working on an existing app and want to add git remotes to enable manual deploys, the following commands may be useful.
Note that on Heroku, you must always use master as the destination branch on the remote. If you want to deploy a different branch, you can use the syntax local_branch:destination_branch seen below (in this example, we push the local staging branch to the master branch on heroku.
$ git remote add staging https://git.heroku.com/staging-app.git