Newbie Need Help:establishing A Relationship

I have a simle question for you:
I created a employee table:

create table employees (
employeeid int primary key,
lastname nvarchar(20) not null,
firstname nvarchar(20) not null,
birthdate datetime,
city nvarchar(15),
country nvarchar(15)
)

and then I created table orders and established a relationship between them:

create table orders (
orderid int primary key,
employeeid int references employees(employeeid),
orderdate datetime
)

The relationship was created successfully.

But I don't know how to establish a relationship between two tables if they were already exist.
Could you tell me how to do it?
many thanks :p