After you have configured the checkout, you need to make sure it is tested properly. Please open your Magento installation in a browser, add a product to your cart, and then navigate your browser to www.yourstore.com/onestepcheckout and see how it looks.
NB: You need to access OneStepCheckout in the same folder as your Magento store. Also, if you are not using Apache rewrites, you need to include "index.php" part of the URL. Also, if your Magento store uses HTTPS, you must access OneStepCheckout on HTTPS url. Here are some examples:
If the checkout looks good and everything seems to be working, please '''make a few test purchases with your different payment methods and options'''. You should make a test purchase for each of the active payment methods you have in your store.
When you have made a couple of test orders and are happy with the checkout - you can activate it so it becomes live for all your customers. You can do this by going to System -> Configuration -> OneStepCheckout. At the top there is a setting for Activate OneStepCheckout. Set this to yes, and the new checkout is now live in your store.