Fold the edges of the slashes in. This should be as scantily as possible, but enough to prevent fraying.
Take a small scrap of the “puff” material that is slightly longer and wider than the slashes. Keep in mind the wider the material that more fabric that can be pulled through to puff. Using thread that matches the trim as closely as possible, stitch the puff material in place while widening the slash as much as desired. I did not want overly pronounced puffing, so mine are fairly narrow, roughly a half to three quarters of an inch.
The addition of the “puffs” distorts the fabric and, quite frankly, is not really puffy. Therefore, turning the piece over, I whip stitch the edges of the inserted puff to pull the trim fabric back to roughly the same size it was before I cut and folded the edges to allow for the puff. When turned right side over, there slashes are back to a smaller size and the puff sticks up through. This can then be manipulated as desired. The trim is then tacked into place as appropriate.