I've had this tank for almost 5 years now. I recently replaced it so wasn't afraid to refashion it.
Also had this other plain white tank that was older than your Mom. I decided it could go too.
So I cut along the side seam and laid it out flat. Then cut off the top part (straps, ect)
Then cut the whole tank into 3 inch strips.
So I ended up with eight 3 inch strips. I took them to the machine. Turned up the tension all the way (Thank you Sarah for teaching me this) and the stitch length all the way down. This way it makes a ruffle.
Which made the strips too short, so I pulled them apart slightly. Start in the middle and work your way out or you will just pull it all out completely.
When it was sewn on I made a mark to see where the next ruffle would go. I wanted them to overlap.....but honestly in the end, everything was pretty wonky :)