In your admin panel go to Order Processing > Shipping Manager. If you just want to set a flat rate for shipping, regardless of the amount of items the customer orders, you will want to add a custom shipping option.
Under 'Add a New Custom Shipping Option' fill in the name for the new shipping method ('Shipping' would probably work just fine). Set the sort order to 0, and add a short description of the shipping option in the Notes area if you would like. Click Insert and the option will be created.
Once the page refreshes, click on the Edit Weights button to the right hand side of the new shipping option. On this Edit Weights page set the Weight Low to 0, Weight High to 999999, Cost to whatever you will be charging for shipping, and the drop-down box to 'Entire Basket.'
This new option will charge what you place in the Cost field for shipping as long as the total weight of the items in the customer's shopping cart is less than 999999 lbs.
