Let's say you make some changes on your master branch and commit a few times. So when you do a good old 'git status' you may have something like this

kongy@Deadpool:~/Development/Sentia/project (master) $ git status
# On branch master
# Your branch is ahead of 'origin/master' by 3 commits.

However you now realize you have taken the red pill. You are in the middle of a monster feature, and realise that you should have branched before. Luckily we can fix this using some git magic.

"Why, oh why, didn't I take the blue pill?"

Let's go back 3 commits by checking it out.

kongy@Deadpool:~/Development/Sentia/project (master) $ git checkout HEAD~3
Note: moving to 'HEAD~3' which isn't a local branch
If you want to create a new branch from this checkout, you may do so
(now or later) by using -b with the checkout command again. Example:
  git checkout -b <new_branch_name>
HEAD is now at 978b493... Merge branch 'master' of github.com:Sentia/project

Nice! Now our repository is sitting pretty at 3 commits previously. And even better, git also tell us how to create a branch from this point. So let's do that now push the newly created branch up to Github.

kongy@Deadpool:~/Development/Sentia/project ((no branch)) $ git checkout -b magic
Switched to a new branch 'tiling'
kongy@Deadpool:~/Development/Sentia/project (magic) $ git push origin magic
Total 0 (delta 0), reused 0 (delta 0)
To git@github.com:Sentia/project.git
 * [new branch]      magic -> magic

Now that we have created and saved the branch, lets grab all those changes in master, merge them into our branch, and push up to Github

kongy@Deadpool:~/Development/Sentia/project (magic) $ git merge master
Updating 978b493..8a74bd3
Fast forward
 project.rb                 |  443 ++++++++++-----------------------

Cool, so our branch is now up to date. Now all we have to do is fix our master branch to become the same as the the GitHub origin master branch. I think there is probably an easier way to do this but I went and got the SHA commit key for the HEAD of origin to know where I wanted to go back to.

kongy@Deadpool:~/Development/Sentia/project (magic) $ git checkout master
Switched to branch 'master'
Your branch is ahead of 'origin/master' by 3 commits.
kongy@Deadpool:~/Development/Sentia/project (master) $ git reset --hard 978b493631a1443b1c84
HEAD is now at 978b493 Merge branch 'master' of github.com:Sentia/project

And you're done. Now you can work in peace on your branch without disturbing your fellow developers. And then cry when you need to merge their changes into your branch.