Here are the steps (from version 4 of WebStorm from JetBrains).

1. Install Node JS

2. Install the node package (using npm) for less (remember where you install this as you’ll need the path later):

npm install less

3. In WebStorm, File > Settings, then under the IDE Settings heading, select External Tools.

4. Click on the + icon to add a new tool:

5. You’ll see a dialog very much like this:

6. Fill it out like this (substitute whatever you’d like of course):

I chose not to have the console window open every time the compilation occurs.

The only things you’ll need to change is the path to the file lessc. That file is actually JavaScript file (minus the extension). So, replace the first quoted parameter in “Parameters” above with the location that you installed the package for less in.

As it’s a JavaScript file you need to be executed, the “Program” is set to the node executable. Provide the full path (when I didn’t, it sometimes wouldn’t work and would complain that a path didn’t exist).

"###YOUR PATH TO LESSC###\lessc" $FilePath$ "$FileDir$\$FileNameWithoutExtension$.css"

I tried using the Working Directory option and it also worked inconsistently with Node. So, I used the options you see above and it seemed to work more consistently.

7. I also added a shortcut so I could execute it on demand:

I picked Ctrl+Alt+L, as it was memorable (and available!).

While this is not using a file watcher technique (you could do that if you’d like instead), it works well enough for my workflow.