A by no means as good as bike fit way to set your saddle at the correct height:
Get someone to hold you on your bike upright, get them to face you holding on to the handle bars with the front wheel between their legs.
Put your pedal down to the lowest point and place your head above the pedal spindle so your leg is locked out. Now when the ball of your foot is above the spindle you should be at the correct leg extension.
With no one knowledgeable with you, perhaps try putting both feet with the heals above the spindle and back pedal and ensure your hips are not rocking.
A by no means as good as bike fit way to set your saddle at the correct height:
Get someone to hold you on your bike upright, get them to face you holding on to the handle bars with the front wheel between their legs.
Put your pedal down to the lowest point and place your head above the pedal spindle so your leg is locked out. Now when the ball of your foot is above the spindle you should be at the correct leg extension.
With no one knowledgeable with you, perhaps try putting both feet with the heals above the spindle and back pedal and ensure your hips are not rocking.