1. Firstly check if you've installed Node.js & Grunt in your machine.

Go to your Magento 2 installed directory.

Execute following command in your terminal or cmd.

npm install -g grunt-cli

Rename package.json.sample to package.json.

Then execute following command.

npm install

2. Make sure your dev/tools/grunt/configs/themes.js has proper configuration.

Then run following commands.

grunt clean

grunt exec:<grunt theme name>

grunt less:<grunt theme name>

For example

grunt clean

grunt exec:luma

grunt less:luma

Then afterwards

grunt watch

1. Firstly check if you've installed Node.js & Grunt in your machine.

Go to your Magento 2 installed directory.

Execute following command in your terminal or cmd.

npm install -g grunt-cli

Rename package.json.sample to package.json.

Then execute following command.

npm install

2. Make sure your dev/tools/grunt/configs/themes.js has proper configuration.

Rename Gruntfile.js.sample to Gruntfile.js.

Then run following commands.

grunt clean

grunt exec:<grunt theme name>

grunt less:<grunt theme name>

For example

grunt clean

grunt exec:luma

grunt less:luma

Then afterwards

grunt watch