diff --git a/doc/usage.rst b/doc/usage.rst
index 27d96ede..30870c6d 100644
--- a/doc/usage.rst
+++ b/doc/usage.rst
@@ -62,7 +62,11 @@ system:
* `Python `_ with pip and virtualenv
* `Doxygen `_
-* `Less `_ with less-plugin-clean-css
+* `Less `_ with ``less-plugin-clean-css``.
+ Ubuntu doesn't package the ``clean-css`` plugin so you should use ``npm``
+ instead of ``apt`` to install both ``less`` and the plugin::
+
+ sudo npm install -g less less-plugin-clean-css.
First generate makefiles or project files using CMake as described in
the previous section. Then compile the ``doc`` target/project, for example::