It takes repeated practice to override the bodys auto motor controls - frustration is your biggest enemy here so try to get yourself into a mellow state of mind before practicing for best chances of success.
As says, it helps to visualise the main muscles and imagine switching off your control of them one at a time. Deep breathing and timing the relaxing with the out breaths seems to work for many.
Patience and sometimes a tutor are your best chances of success I think.
