Check: Design of Analog Fuzzy Logic Controllers in CMOS Technologies: Implementation, Test and Application by Carlos Dualibe, Michel Verleysen and P. Jespers (Mar 24, 2013)
It is good to start by collecting related literature and get a clear understanding of how defuzzification algorithms work. Once you have completed this, you will be able to come up with an architecture that can be implemented on a hardware platform. Wish you all the best.
For any logic you want to implement in FPGA, first you need to formulate your own algorithm or find out the best algorithm available in literature related to your work. Then try to design an architecture which maps your algorithm in to hardware. Once you are done with an optimized architecture, you can program the FPGA by writing a HDL code (Verilog/VHDL) for the proposed architecture. Good Luck !!
Check: Design of Analog Fuzzy Logic Controllers in CMOS Technologies: Implementation, Test and Application by Carlos Dualibe, Michel Verleysen and P. Jespers (Mar 24, 2013)