FixieConventions.cs

public class CustomConvention : Convention
{
    public CustomConvention()
    {
        Cases.Where(StartsWith("should", "it"));

        FixtureExecution.CreateInstancePerFixture();

        InstanceExecution.SetUpTearDown(StartsWithWhen(), StartsWithAfter());
    }

    static MethodFilter StartsWithWhen()
    {
        return new MethodFilter().Where(StartsWith("when"));
    }

    static MethodFilter StartsWithAfter()
    {
        return new MethodFilter().Where(StartsWith("after"));
    }

    static Func<MethodInfo, bool> StartsWith(params string[] prefixes)
    {
        return method => prefixes.Any(s => method.Name.StartsWith(s, StringComparison.OrdinalIgnoreCase));
    }
}