When using the git-flow extension library, executing
git flow init on an existing repo will create the develop branch:
Admin:~/environment/gitflow-workshop (master) $ git flow init Which branch should be used for bringing forth production releases? - master Branch name for production releases: [master] Branch name for "next release" development: [develop] How to name your supporting branch prefixes? Feature branches? [feature/] Release branches? [release/] Hotfix branches? [hotfix/] Support branches? [support/] Version tag prefix? 
List the current branches. The current branch will be highlighted in green and marked with an asterisk.
Admin:~/environment/gitflow-workshop (master) $ git branch * develop master
Push commits made on a local branch to a remote repository.
Admin:~/environment/gitflow-workshop (develop) $ git push -u origin develop