Lets say there is a project called python and you want to contribute. So you should fork python project and ALWAYS create a separate branch for the patch/feature you are working on and NEVER commit on the master branch of forked repo.
Lets call your forked repo as python-forked.
Once you fork a project, add a git remote called upstream (or whatever name you feel like using), which points to original repo. This remote will help you keep your project updated and in sync with original repo (from where you forked).
$ cd python-forked
$ git remote add upstream https://github.com/guido/python.git
Consider 3 scenarios.
The simple, fork and send PR
Create a new branch, name it on the patch/feature you are working on:
$ cd python-forked
$ git checkout -b bugfix-unicode-strings
Work on bugfix-unicode-strings and make all the changes you want. And then do a push to your github account, which is usually origin remote:
$ git push origin bugfix-unicode-strings
And then send PR, to master branch of guido/python, with your branch bugfix-unicode-strings.
Now, tomorrow, guido may add new features and you might want to update your forked repo. It’s simple, just pull from the upstream to master branch of python-forked
$ cd python-forked
$ git fetch upstream
$ git checkout master
$ git rebase upstream/master
Update and PR
You have forked the project and maintainer has later moved on and added new features which you need in the current patch you are working on
You need to fetch the new changes from upstream and put those in your patch branch. While doing this, usually I update my master branch also:
$ cd python-forked
$ git fetch upstream
$ git checkout master
$ git rebase upstream/master
$ git checkout existing-patch-I-am-working-on
$ git rebase master
You could also do $ git rebase upstream/master in last step to update the current patch branch.
Update, resolve conflicts and PR
You have forked the project and maintainer has made some changes to the file you are also working on
Fetch the changes and merge it with current patch branch you are working:
$ cd python-forked
$ git fetch upstream
$ git checkout master
$ git rebase upstream/master
$ git checkout existing-patch-I-am-working-on-which-has-a-file-edited-by-guido
$ git rebase master
above rebase will fail(?) (or interrupted) and terminal will ask you to resolve the conflicts and then merge.
usually:
# solve the conflicts
$ git rebase --continue