how to make a costas receiver?

Hello,
is any one can guide me to built a costas receiver. At the input of a mixer the LO and RF signal creates an IF signal. This IF is the frequency difference of LO and RF irrespective of their position of frequency spectrum, i mean IF=LO-RF or IF=RF-LO. I need to track this IF signal at the output of the mixer. I want to use this costas receiver to track the position of the RF using the I (inphase) and Q (Quadrature) component.

any idea! thanks in advance

Regards
Reza