To set up flat rate shipping in Shopify, follow these steps:

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

2. Click on "Shipping and delivery" from the list of options.

3. Click the "Manage rates" button in the "Shipping rates" section.

4. Click the "Add rate" button to create a new shipping rate.

5. Enter a name for the shipping rate in the "Name" field. This can be anything you like, such as "Flat rate".

6. In the "Price" field, enter the flat rate you want to charge for shipping.

7. Under "Shipping zones", select the zones that this shipping rate will apply to. You can choose to apply it to all zones, or specific zones based on their country or region.

8. Click the "Save" button to save your changes.

9. Repeat these steps to create additional flat rate shipping rates if needed.

10. Test the shipping rates by adding products to your cart and going through the checkout process.

Note: Flat rate shipping applies the same shipping cost to all orders, regardless of the size or weight of the items being shipped. If you want to charge shipping based on the weight or size of the items, you can use Shopify's built-in weight-based or price-based shipping options.