A basic QueryOver query is performed against an ISession using the QueryOver<T> method, where T is the type of a mapped entity.

IList<Customer> customers = session.QueryOver<Customer>()
    .Where(c => c.LastName == "Simpson")
    .List();

To join and and for instance filter on the joined table use JoinQueryOver.

IList<Customer> customers = session.QueryOver<Customer>()
    .Inner.JoinQueryOver(x => x.Organisation)
    .Where(y => y.Name == "Acme Inc")
    .List();

It's possible to use JoinAlias method to join several tables. It's useful when it's needed to specify some property from the joined table in the select statement:

Customer customerAlias = null;
Organization organizationAlias = null;

IList<Customer> customers = session.QueryOver(() => customerAlias)
    .Left.JoinAlias(x => x.Organization, () => organizationAlias)
    .Where(customer => customer.Name == "Customer Name")
    .And(() => customerAlias.Age > 18)
    .AndNot(() => organizationAlias.Name == "Forbidden Organization")
    .List();