Here's example of actual embroidery on crocheted pillow but same could be done on knitted fabric. Draw design on tissue or light interfacing (use this if fabric needs stabilizing), baste to fabric. Stitch directly thru the tissue/interfacing as it'll tear away when done. I unplied worsted and used large tapestry needle.
Dupe stitch is, as noted, another option, but you're limited to the shapes created by replicating existing sts.
Keeping to the knitting basics