You don't mention which routers you are using, but if we assume something like a 7300 you could use an STM-4 POS interface such as the 7300-1OC12POS-MM. Just remember that this comes in different flavours depending on how the SDH service is presented by the carrier.
Choices:
Short reach: MM 1310nm (eg 2km)
Intermediate reach: SM 1310nm (eg 15km)
Long reach: SM 1310nm (eg 40km)
The higher the range, the more optical power available (if you need it), and the higher the cost. Rule of thumb is to check it out first and buy only what you need.
This applies to the 7300, but similar capabilities are available on the 12000, and 7600.
Then have a look at the configuration guide and see what else you need to know.
eg you will need to set framing to SDH (default is SONET)
http://www.cisco.com/en/US/products/hw/modules/ps2710/products_module_installation_guide_chapter09186a0080104aa5.html
