if you want to insert the data from the two tables (header and detail) to test table you can do something like this...
1. get the inserted value in dbo.inserted
2. join dbo.inserted and Header Table.
sample code below
CREATE TRIGGER Detail_Insert
ON detail
FOR INSERT
AS
insert into Detail_table
select I.id, * from dbo.inserted I
inner join header H
on I.id = H.Id
This code will work. Just make some neccessary adjustment as to what fields to you want