If you've bought the product at Magento's Marketplace - use Marketplace installation instructions
Otherwise, you have two options:
- Install the sources directly from our repository - recommended
- Download archive and use manual installation

For developers

Use this approach if you have access to our private repositories!

cd <magento_root>
composer config repositories.cssoft composer http://cssoft.github.io/packages/
composer require cssoft/checkout-fields:dev-master --prefer-source
bin/magento module:enable\
    CSSoft_Core\
    CSSoft_Checkout\
    CSSoft_CheckoutDetails
bin/magento setup:upgrade