You can definitely put trim anywhere you want. Pin it to where you want it, and then just stitch on top of it to attach it. If the trim is is really narrow you can just stitch down the center. For wider trim you can stitch on each side. For doing the corners, you might want to try doing something similar to putting on binding. You sew it until you get to where you want the corner to be. Cut your threads but don’t cut the trim. Take the trim and fold it so it’s now going perpendicular and continue sewing. You can do it!