The simplest way to mount the block is to first tie the stropping line around the metal horse, then wrap it around the block and tie it off around the block. Remember that the sheave hole should be closest to the horse.
I took a slightly more complicated route, which is to take a line and loop it around the horse. Then, I made a simplified seizing, by using thread and tying a simple overhand double-knot. The knot is then slid down close to the horse to tighten the loup. A tiny dab of glue is applied to the knot to hold it in place. When dry, I can take the loose ends and make a loop, insert the block, and tie a simple overhand knot around the block. Below shows an example using a piece of wire to attach the block to.