Before making changes, in the project directory checkout to a new branch with name of your choice
git checkout -b {branch_name}
Start adding/modifying the code.
Once you are done making changes
Create a new folder with name 'Preview' in the same project folder you have made changes to and add a preview image of the website/page in the 'Preview' folder.
Add the files you want to commit to the staging area.
Now you will be able see a button named "Pull request". Click on it.
Add appropriate message and click the "Create Pull Request" button.
Resolving errors
If you get a message like "This branch is X commits ahead, Y commits behind" then visit this thread and follow the steps. Also don't forget to push and pull from proper branches.