This branch is X commits ahead, Y commits behind

There is a easier way to sync your fork with the original repository. Open your git:

git remote add upstream https://github.com/upstream/repo.git

When you want to sync the changes from upstream

git pull --rebase upstream master
git push --force-with-lease origin master

I highly recommend that you make new branch for each changes, so you can work on something else while other PR is still waiting for approval.

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

git pull

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

git checkout -b [name_of_your_new_branch]

Push the branch on github :

git push origin [name_of_your_new_branch]

I use GitHub Desktop which is easy to use for overall.