The Acala wiki provides a guide, but it's a touch techy, like many things on the Polkadot Ecosystem, so I thought I'd provide a simple guide.

• Navigate to Karura's apps.
• Click on the settings wheel in top right and select "Flexi Fee".
• Define an appropriate token as your default fee token and save.
• In the side bar, if not already selected navigate to your portfolio, which is the top link.
• In the portfolio screen, which has already opened, select "Cross chain".
• Select "Inter Kusama Transfer" - which will present the below 
  
  

• Make sure you have selected Kusama as the source chain and Karura as the destination chain.
• Under the "To Account" tab make sure to pick your KSM account not your DOT account.
• Enter the amount you want to transfer, check fees, and click transfer
• Enter your Polkadot.{js} password and confirm
• The KSM should appear in your Karura account in the blink of an eye