Adding crochet borders

I just finished knitting my first sweater. The pattern calls for two rows of dc on the bottom of the sweater body & on the bottom of each short sleeve. I’ve learned how to crochet & how to do the dc stitch, but can’t figure out how to add the border. Do I need to do a chain first or use the existing bottom of the sweater instead & just pick up the stitches as if they were a chain? I also can’t come up with a way to add the 2nd row of dc. HELP!! Thanks in advance.

The bottom edge of the sweater/sleeve is the foundation. I would DC in each stitch for the first row and then DC into the DCs for the second row. Double-crochet needs a 3-chain “turning chain”, so I’d ch 3 at the beginning of each row. If DC into every stitch is too crowded, DC every other stitch.