Magento – How to set up flat rate shipping

In the case where you want to have the customer choose from multiple options, I recommend using one of WebShopApps matrix plugins. Here's a good one:

http://www.magentocommerce.com/magento-connect/webshopapps-matrixrate-1-multiple-table-rates-extension-certified-bug-free.html

Otherwise, to get an automatic calculation based on value, use table rates to achieve what you're looking for - to get an example of a table rates CSV:

For the £50 free delivery option, use free shipping shopping cart price rules.

_{Source: http://www.magentocommerce.com/knowledge-base/entry/how-do-i-set-up-table-rate-shipping}

here is a nice tutorial on how to create a shipping method.
Or this one from Inchoo.

Basically you have to follow the standards presented in those tutorials and all your logic goes into the collectRates method of the shipping model.
There you can calculate the costs of your shipping based on the values you receive in the $request parameter and decide if the method is available or not and set a price to it.