Create a basic Gruntfile with grunt-init.
If you haven't already done so, install grunt-init.
Once grunt-init is installed, place this template in your ~/.grunt-init/ directory. It's recommended that you use git to clone this template into that directory, as follows:
git clone https://github.com/gruntjs/grunt-init-gruntfile.git ~/.grunt-init/gruntfile
(Windows users, see the documentation for the correct destination directory path)
At the command-line, cd into an empty directory, run this command and follow the prompts.
grunt-init gruntfile
Note that this template will generate files in the current directory, so be sure to change to a new directory first if you don't want to overwrite existing files.