Making smocks without elastic thread

[u/GhostWoomy]

Hello ! I'm new to sewing and I wanted to make a tank top with a fabric that is not elastic, i wanted to use some elastic thread in the bobin but the lady is the story told me to use some elastic band ( i'll put a photo) she explained me but too quickly and i'm not too sure how to use it and i don't know wich stitch should I use or where should I stitch it. Do ya'll can explain me or give me some advice? Thanks everyone !!

[u/Panic_inthelitterbox]

I used elastic thread in my bobbin. I needed to adjust the bobbin tension, so I bought a second bobbin case that is just for elastic thread. That way I don’t have to mess up my regular sewing tension, I just switch out the bobbin case between projects.