How to convert a 16 bit value to signed 12 bit value ?

Background:

Using Simulink:

1. I combined two unsigned data bytes, 8 bit from first databyte and 4 bit from the second databyte(masking and shifting operation is done) and i recieve an output as 16 bit unsigned value. This value needs to be converted into a signed 12 bit value

I tried using "Data Type Conversion" , fixedpoints but was unable to achieve the result. Please guide how to do this operation in simulink

Thanks in advance