In your github fork, you need to keep your master branch clean, by clean I mean without any changes, like that you can create at any time a branch from your master. Each time, that you want to commit a bug or a feature, you need to create a branch for it, which will be a copy of your master branch.

When you do a pull request on a branch, you can continue to work on another branch and make another pull request on this other branch.

Before creating a new branch pull the changes from upstream, your master needs to be up to date.

Create the branch on your local machine and switch in this branch :

$ git checkout -b 

Push the branch on github :

$ git push origin 

When you want to commit something in your branch, be sure to be in your branch.

You can see all branches created by using :

$ git branch

Which will show :

* approval_messages
  master
  master_clean

Add a new remote for your branch :

$ git remote add  

Push changes from your commit into your branch :

$ git push origin 

Update your branch when the original branch from official repository has been updated :

$ git fetch 

Then you need to apply to merge changes, if your branch is derivated from develop you need to do :

$ git merge /develop

Delete a branch on your local filesystem :

$ git branch -d 

To force the deletion of local branch on your filesystem :

$ git branch -D 

Delete the branch on github :

$ git push origin :

The only difference is the : to say delete.

If you want to change default branch, it's so easy with github, in your fork go into Admin and in the drop-down list default branch choose what you want.