You can use signs over signs, learn rules for it, and sometimes get the right answer by this mean - it's probably what a professor expects from you, and then do it.
Alternately, you can forget all signs and think. In the reaction to be computed, what is the effect of every heat of formation?
The production of H2O releases heat: 6*241.8kJ (all gaseous)
The destruction of NH3 absorbs heat: 4*45.9kJ (all gaseous)
So the reaction releases 6*241.8-4*45.9=1267kJ.
You just have to be clear whether the formation of a particular compound has absorbed or released (Hf<0) heat and account it properly in the reaction to be computed.
This is less systematic than formal operations of signs, possibly less academic hence more difficult to teach, but I make far fewer mistakes that way - especially with heats of formations with so many signs on every side.