Wash the acorns in some water to get the surface dirt off.

Bake them in the oven on the lowest level of heat for 1-2 hours.  

Use a tiny flathead screwdriver to pry off any of the acorn caps.

Match them with a similar size acorn.

Use wire cutters to clip off the stem of the top of the cap.

Make a hole in the top with a 7/64 inch drill bit.

Thread one end of the cord through the acorn cap from the top.

Secure with hot glue.

Make the cord length as long or as short as you want to wrap around the napkin a few times. Let dry.

Make a knot at the top side of the cap. 

Glue the nut to the cap.

Attach the other end of the cord to the first acorn's mate.