What volume of solution do you want?
The typical way to make a buffer is to make the concentration of buffer you want. I.E. make 500mL of 100mM Ammonium Acetate. Then adjust the pH with an appropriate acid or base (depending on which direction you want to go), and measure the change with a pH meter, and just adjust to the correct pH.
Yes, you can solve for it using the Henderson-Hasslebalch equation, and it is a good start. But it is only a starting estimate or an idea of how much you will have to add; it is more complicated the the Henderson-Hasslebalch equation leads you to believe. Rest assured, that (if done correctly) what you calculate using the Henderson-Hasslebalch equation will be pretty close in this case. But the best method is to just adjust the pH with a pH meter.
Alternatively, you can download Boreks buffer program and use it; thought I do not know if the demo version does ammonium acetate, but it uses a little more complicated math then the Henderson-Hasslebalch equation:
http://www.chembuddy.com/?left=Buffer-Maker&right=buffer-calculatorhttp://www.chembuddy.com