Roll out cookie dough to 1/4-inch thick.
Use templates and paring knife or cookie cutters to make 3 deer and 5 trees (use one of the tree templates twice).
With skewer or large pastry tip, make a hole in each cookie as marked on templates.2.
Chill and bake cookies according to recipe.
Let cookies cool completely.3.
Fill pastry bag with royal icing.
Squeezing gently from the top of the bag, start just above deer cookies' surfaces and pipe dots for eyes; use a zigzag motion to create shadows on their undersides.
Fill in ears of smaller deer, and make dots along deer backs.
Pipe an outline on trees.
Let icing dry at least 2 hours (preferably overnight); flip cookies, and pipe icing around their perimeters, then fill in.
Let icing dry, as above.4.
Paint embroidery hoop, and let dry.5.
Fold 1 length of baker's twine in half, and pass loop through hole in cookie, from back to front; pass snipped ends through loop, and draw them upward.
Repeat with remaining cookies and baker's twine.6.
To suspend cookies, start with 2 similar-size cookies at opposite sides.
Tie snipped ends of twine around hoop, but do not knot.
Adjust placement and twine lengths to balance cookies; this is easiest to do if a second person holds the hoop.
Repeat, 2 cookies at a time.
Add last pair, moving them as needed.
Knot strings once balance is achieved.7.
Screw hook into the ceiling.
Double-knot the 4 lengths of ribbon to hoop at equal intervals.
Gather ribbon pieces, and tie them together around the hook, leaving 1 foot of ribbon to make a bow.
If mobile isn't balanced, shift ribbons slightly.