The trachelium shown in the tutorial is not as fully bloomed out as the trachelium in this picture below.
Handle it gently, as the flower heads can wilt it you treat it too roughly.
She tucked clusters of it between the other flowers, using it as a nice filler with a great texture element.
Now it becomes a matter of filling in with the remaining flowers. Remember to cluster some in groups if you wish to retain the "English Garden" feel to the bouquet.
The flowers in the center should stand straight up, then begin angling the flowers outward until those touching the collar are at a complete 90 degree angle to the center rose.