Mark tie placement with tailors chalk/erasable pen (mine was 12 cm below bottom of armhole.)
Insert tie, sandwiching it between right sides of front and back pieces.
Ties are usually inserted into side seams before the side seams are sewn.
If you've already sewn the side seams as shown in the photo:-
Unpick about 2.5cm (1") NB: No need to unpick overlocking/zigzag. Insert tie and restitch gap securely, going forwards and backwards over the tie.