1. Clone your fork:

git clone [email protected]:YOUR-USERNAME/YOUR-FORKED-REPO.git

2. Add remote from original repository in your forked repository:

cd into/cloned/fork-repo
git remote add upstream git://github.com/ORIGINAL-DEV-USERNAME/REPO-YOU-FORKED-FROM.git
git fetch upstream

3. Updating your fork from original repo to keep up with their changes:

git pull upstream master

4. Replay your commits on top of the new commits from the destination branch so that the merge can be a ‘fast-forward’.

git rebase master

5. Update from remote.

git pull

6. Checkout from fork

git remote add theirusername https://github.com/theirusername/reponame
git fetch theirusername
git checkout theirusername/theirbranch
or create new branch mynamefortheirbranch
git checkout -b mynamefortheirbranch theirusername/theirbranch

7. Squashing Git Commits into one

git checkout master
git merge --squash bugfix
git commit
or
git commit --signoff
or
git checkout master && git pull
git merge feature_branch
git reset origin/master
git add . --all
git commit

8. Set permissions to file

According to official documentation, you can set or remove the "executable" flag on any tracked file using update-index sub-command. To set the flag, use following command:

git update-index --chmod=+x path/to/file

To remove it, use:

git update-index --chmod=-x path/to/file

Starting with Git 2.9, you can stage a file AND set the flag in one command:

git add --chmod=+x path/to/file