The letters SD are embroidered with metal thread on black cloth. Twisted silver cord edging. The size of the rhombus is 65x50.
How to sew the insignia on? Put the patch on the left sleeve of the service jacket. Fix it with a bright thread (which then remove) or glue. When the patch sit properly, it needs to be sewn with a thread over the edge.
Sicherheitsdienst (from German: security service) or SD, was the intelligence agency of the SS and the Nazi Party in Nazi Germany. Originating in 1931, the organization was the first Nazi intelligence organization to be established and was considered a sister organization with the Gestapo (formed in 1933) through integration of SS members and operational procedures. Its first director, Reinhard Heydrich, intended for the SD to bring every single individual within the Third Reich's reach under "continuous supervision".
Historical information from: en.wikipedia.org