To install Klarna checkout on WooCommerce, follow the next guidelines: 

• Download the Klarna checkout for WooCommerce plugin here. 
• Check  the required versions for configuring the plugin here.
• Check the required settings for configuring the plugin here.
• After you installed and activated the plugin, you can find it in WooCommerce > Settings > Payments > Klarna Checkout.
• Use Test mode to test your integration and ensure it works properly before taking real orders. You can sign up for test credentials here.
• Generate your Klarna API credentials in the Settings section of the Klarna Merchant portal. The Klarna API credentials consist of an API username and password.
• Enter the generated API credentials in the appropriate section (Production or Test). Note that Klarna API credentials work either for Production or Test, but they do not work in both Production and Test environments.
• Promote your partnership with Klarna by adding On-site messaging. Let your customers know about deals and flexible payment options throughout their purchase journey. More information about On-site messaging for your WooCommerce store is available here.

For detailed documentation of Klarna checkout for WooCommerce, see here.

In summary:

• To install Klarna checkout on your WooCommerce store, you have to download the Klarna checkout plugin, configure the required versions, and activate it in WooCommerce settings. 
• You can test your integration using Test mode before managing real orders.
• For generating Klarna API credentials, go to the Merchant portal settings.