Ok, we know tetrahedron is a sturcture formed by four units in which three units represent base and one to the top. So remember that tetahedral voids are formed when such a situation arises. Octahedral voids arise when two triangles are placed with their apex faced opposite sides.
So memory tip:
Place three balls (this is first layer). Here apex faces north (lets say)
I place a ball from the second layer (which shall be in top of first) on the void which is created by the three balls in first layer, oh well boy! you have a tetrahedral void. Accordingly all the rest units from the second layer will arrange itself (you can just draw after making this first tetrahedral)
If in the second layer placing, I place three balls such that apex of second layer balls (3 in no.) faces opposite (in our case it faces south) such that center of the triangle from layer I and II lie on same axis (just place on exact top) we get a void in between called as OCTAHEDRAL void.
Additional Information which may be helpful:
Let the number of close packed spheres be N, then:
The number of octahedral voids generated = N
The number of tetrahedral voids generated = 2N
