I'm a 34D in a sports bra and those stupid shelf bras are worthless lol...I hack em out all the time!

It helps if you have a pair of decent sewing scissors that are sharp, with a slightly longer blade, and nice points for tips (to get in the tight space of cutting out the shelf bra). These can be purchased reasonably at somewhere like Walmart in the craft/sewing section.

I personally have tried leaving the fabric and cutting the band only, as discussed, and for me I did not like it. Especially if you are moving to work out, it will ride up and around in places you do not want it to be without the bottom secured. So just cut it all out...except maybe in the shoulder strap areas...just leave a straight across line where this section begins.

The biggest tip I can give you is to not be in a rush. Just when you have a few quiet mindless minutes to kill use a nice clean flat surface, like a table or kitchen counter, and carefully trim it out...checking each few snips that you have not snipped the actual top, versus the bra part.

Good Luck!