How is distance in light-years converted to years back in time? When I look at the Alnitak,the left most star of Orion's belt, it is 736 light years away. How could I convert this distance to an estimate of how long ago what I am seeing happened.

Would it be simply be 736 years ago?

Are there instances of when this isn't as straight forward?
Yes, for stars in our galaxy, if a star is 736 light years away, the light took 736 years to reach here.

For very very distant objects, you need to account for the expansion of space. The light might have been travelling for 10 billion years, but in that time space has expanded and so the distance that you would find if you froze time and measured would be greater (to get the proper distance at the current time) it would be larger.  This effect is not significant at distances less than about a billion light years.