How do you add to a completed project

I have knitted a small square to add to a shadow box. It is completed, I did the bind off and all. But, now I want to add to it. How can I add a trim of another yarn color to a completed project?

Thank you.

Pick up stitches

Thats what I thought so I watched the video on here, it didn’t make sense. I will check on you tube. Thank you.

See if this tutorial helps. It shows picking up sts along the cast off or cast on edge and along the side edges.