Less 7.x-3.0 documentation can be read here: LESS 7.x-3.0
1 of 3 possible engines is required.
You can read engine installation details here: LESS Engines
You must select which engine you are using on 'admin/config/development/less'.
Please read here for details for usage here: LESS Module Usage
You can modify variables within your .less files using hooks provided by the LESS module.
You can read the details here: LESS Variables