You’ve got two options at this point. You can use your scissors to trim the excess fabric in half and finish with a fancy French seam, like in this video:
Or, if you’re eager to wear your newly adjusted tank, and you own a pair of pinking shears, you can pink the excess fabric and be ready to rock your tank top!
The French seam will definitely look fancier, but unless the tank is made from pretty heavy fabric, pinking shouldn’t affect the finished product’s comfort at all.
Here’s my new, size medium tank top: