First my pile of materials. (Not shown is some kind of trim for the edges.)
I cut my fabrics about 1 inch bigger than the intended size. (I add about a quarter of inch around the embroidery.) I have two rectangles that I cut from mat boards, the kind used to frame pictures. Also, I use white craft glue, a piece of quilt batting, masking tap, and an exacto knife and clear ruler to cut the mat board.
I first glue the quilt batting to one of the cardboards. This will be for the front. Let it dry all the way, or you risk the glue seeping out to your embroidery, and that's not so nice.
Next I use the tape to attach the fabrics to the mat board. This isn't the fanciest of ways, but it works well. It can be annoying lining the design up nicely, but it's worth the time to make sure it is. I tape down the corners, then the sides. Pull firmly on the fabric, so isn't loose on the front side. If you are a perfectionist you can make perfect mitered corners. It will make the corners less bulky if you do that, but the trim can hide some of the imperfections in the corners.
I do the same for the embroidered piece, using a bit more tape to hold it down evenly. Note, make sure the tape doesn't get too close to the edge or it will be hard to glue the pieces together in the next step.
Next is to glue the two pieces together. Spread a layer of glue around the back side of your covered mat boards. Make sure it goes to the edges and corners, but preferably not over, especially if you plan to sew your trim in place. I put the two pieces together, then put the ornament down on a flat surface face up. I cover the embroidered surface with a folded towel, and put on some heavy books. Then let this sit until it's all dry.
Now it's time to attach the trim. You can choose to glue it on or sew it on. You can use pretty much any kind of trim, yarn, ribbon, or beads that you want. In this example, I couch down some soft cotton yarn. I like to leave my stitches a bit uneven.
I tuck the ends of the yarns a bit in between the two pieces to keep them out of the way.
I like to use beaded hangers for my ornaments. I usually use 2 strands of cotton floss and attach it firmly will several small stitches, then add the beads, and make several small stitches again before cutting the extra floss. And that is pretty much it! It isn't hard or time consuming. :)