Navigate to Applications > Utilities > Terminal.

defaults write com.apple.dock persistent-apps -array-add '{tile-data={	|; tile-type="spacer-tile";	|'

Then, paste the following and press enter:

killall Dock

The dock will now refresh itself and a spacer will appear. You can move it by dragging it around with your mouse. For additional spacers, simply repeat the step above. To remove a spacer, right click it and select "Remove from dock".