The Clock Face is probably the easiest means of describing clearly where the feet should be placed in any particular stance, as most people learning Katas know how the hands of a clock work

The Centre point of the Clock
 

will always relate to the Back foot of the stance 
i.e. the foot furthest away from the direction of the current technique.

The numbers of the clock relate to the positioning of the front foot in relation to the back foot.

The 12 will always be the direction in which the Kata starts.  

This means if you are facing towards the 9 o'clock position, you will be facing directly to the Left of your start position 

All examples are Left Side Facing towards the start direction of the Kata
12 O'Clock Position in Back Stance
(Kokutsu Dachi)
10.30 Clock Position in front Stance
(Zenkutsu Dachi)
12 O'Clock Position in Horse Stance
(Kiba Dachi)