# You work on master, then after a while you realize you should have worked in a branch.
# You want to move the last X commits to a branch and remove them from master

# commit what you got, even if it doesn't work
git commit -m "work in progress, doesn't work" -a

# create the new branch - it will have everything master has
git checkout -b MY_NEW_BRANCH

# go back to master and remove the extra commits
# be extra careful if other people have pushed / pulled
git checkout master
git reset --hard HASH_OF_COMMIT # or HEAD^ if you just want to undo the last commit 
                                # or HEAD~3 if you just want to undo the last 3 commits
git push --force origin master

# now carry on working on new branch
git checkout MY_NEW_BRANCH

# if you had commit things that did not work, uncommit, keeping the changes on your filesystem
git reset HEAD^
# ...do work...
# commit working files
git commit -m "now it works" -a

# push it to the remote
git push origin MY_NEW_BRANCH