Adding Record To Multiple Categories

I am pretty new to the DB part of this but have built an asp.net web appplication with 2 tables:
FORMS and UNITS
I have created a web page that will allow users to add forms and associate a unit with that form. I now need to be able to allow users to associate the form with multiple units.
I can change the web page list box to allow multiple selections but that doesn't solve the problem. This seems like a pretty simple task but I can't seem to find anything on it.
any help???
below is the stored procedure I was using:
CREATE PROCEDURE dbo.USP_AddForm

@UNIT_IDint,
@F_TYPE varchar(20),
@Titlevarchar(100),
@Keywords ntext = NULL,
@Descriptionntext = NULL,
@FileNamevarchar(50)


AS
BEGIN

SET NOCOUNT ON
DECLARE @NewFOrmID INT
INSERT FORMS

(
UNIT_ID,
F_TYPE,
Title,
Keywords,
Description,
FileName,
RevDate
)

VALUES
(
@UNIT_ID,
@F_TYPE,
@Title,
@Keywords,
@Description,
@FileName, GetDate()
)

SELECT @newFormID = @@IDENTITY
SELECT newFormID = @newFormID
END


GO