When couching metallic or other threads onto the surface, the ends of the threads need to be sunk, or plunged, to the underside of the fabric. In Japanese Embroidery the tool used for this purpose is called a sinking needle.
To make a sinking needle, first under twist a length of thread (one strand of flat silk). Thread a large needle (size 10 hand made Japanese needle) onto the twisted thread, then fold the thread and over twist the two halves together resulting in a two-ply cord with a needle trapped in fold of the thread.
To prevent the thread from unravelling, fix the thread with glue (the glue we use is called yamato).
Pass the end of the thread through the eye of the needle to form a loop.
To use a sinking needle, insert the needle into the fabric at the point were the couched thread should be sunk (or plunged) to the reverse of the fabric.
Pull the needle through the fabric but leave a loop of thread on the surface of the fabric. Insert the end of the tread to be sunk into the loop.
Continue to pull the sinking needle to tighten the loop against the thread, then gently pull and wriggle the sinking needle to ease the tail through the fabric.
I find it helps to grasp the thread of the sinking needle close to the underside of the fabric (with my left hand) and pull on this rather than to pull on the needle. I also like to support the underside of the fabric with the thumb and forefinger of my right hand either side of the sinking needle.
Sink one thread at a time, and be gentle – a sharp tug works, but it is much better to take your time and ease the thread through.