Install 
Download everyfiles, not only tar-ball.

Do make.

 preparation 
You need to modify hdecay.in for your purpose.

The meaning of some variables are described in hdecay.f.

To modify some formulae for higgs decay, you modify hdecay.f in your own risk.

 How to use 
After modification hdecay.in for your purpose, do

make

This compile hdecay.in. After this do

./run

You will find br.sm1 and br.sm2 which are written branching ratio.

 memo 
SM decay width are summarized around line 3740 in hdecay.f.

CAX: Amplitude contributed from X particle loop.
X = T(top), B(bottom), L(tau), W, TP, BP, EP
  (last three are 4th generation particles.)

CAT: Amplitude contributed from top quark loop with some corrections.
CAT0: Amplitude contributed from top quark loop at LO.
 
If you want to use your own functions and write it in YourFunc.f,
then you should modify makefile, write YourFunc.o in appropriate line.

 Useful links