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