If you already have a working joystick, but do not have a spare axis to assign the rudder keys to, then this solution will work for you.
Once you have loaded into FSX, you have a choice either to a. Use the aileron axis to control your rudder OR b. to use your mouse as your rudder pedals.
For option A, you will need to ENABLE “Auto Rudder” control by pressing CTRL + SHIFT + U (or go into your realism settings) AND go to your right hand side MCDU > Options and Enable “Auto Rudder” from there as well.
For option B, you will need to follow Solution 2