Shopify Scripts is a feature that allows you to customize the behavior of your store's checkout process by adding custom code. Here are the steps to write Shopify Scripts:

1. Log in to your Shopify account and go to the Settings section.

2. Click on "Checkout" from the list of options.

3. Scroll down to the "Order processing" section and click "Manage checkout scripts".

4. Click the "Create script" button to create a new script.

5. Choose the type of script you want to create: "Line item", "Shipping", or "Payment".

6. Enter a name for the script and click the "Create script" button.

7. In the code editor, write the code for your script. The code you write will depend on the type of script you are creating.

8. Save your script by clicking the "Save" button.

9. Test your script by placing a test order and verifying that the behavior of the checkout process is as expected.

Note: Writing Shopify Scripts requires knowledge of programming languages such as Ruby, JavaScript, and Liquid. If you are not familiar with these languages, you can hire a Shopify expert or developer to help you write and implement custom scripts in your store. Additionally, it is important to test your scripts thoroughly before deploying them to your live store to ensure they do not break the functionality of the checkout process.