What is the proper way to attach a top railing to these? I assume I'll have to miter and cut into the railing to get it to fit properly?

McMorty, These connectors are for attaching 24 rails to posts. You can follow the installation instructions on this product page under Overview and by clicking Installation Manual. Measure the distance between two posts and subtract 1/2”. Cut rails to this length. Attach a connector to each end of the rail with two screws through the pre-drilled holes. Set the rail at the desired height and check to ensure it is level. Fasten with screws through the pre-drilled holes. To ensure the connectors stay in place, apply a small amount of clear silicon on the screw being inserted into the connector. -Your Deckorators Team