First thing you need to do is make sure the assembly for your provider is signed, strongly-named and stored in the GAC. Get the Culture, Version and Public Key Token values for your assembly out of the GAC
Edit the file:
Find the trustedProviders section, and add a new line for your provider as follows:
add type="MyCompany.CustomProvider, System.Web, Version=126.96.36.199, Culture=neutral, PublicKeyToken=c14f5f7f11d61b3a
(where the type name, version, culture and publickeytoken are what you pulled out of the GAC)
Save the file
In the web.config file for your site, make sure that the provider is referenced using the fully qualified assembly type name, and you should be good to go.