Exactly where and how is it leaking?
If the leak is inside the sewer pipe, double check that you properly installed the valve & seals and didn't accidentally damaged them during the install. Also make sure they are the correct way (backwards seals is a common replacement error)
But if the leak is on the outside the sewer pipes, you'll have to physically locate the leak point to determine what needs to be replaced which determines the level of difficulty. It could be something as simple as a improperly tightened band connection or as problematic as a cracked pipe junction.