If you carry out the operations in order of priority you have:
100÷20=5; 62*(-1)=-62; {36+21-9}={57-9}=48 because division and multiplication should be done before addition and subtraction, and anything in brackets can be calculated separately. So now we have a simpler calculation:
[5-(-62)-48+91]=[5+62-48+91]=[67-48+91]=[138-48]=90.
You break down the calculation into smaller bits just as you break up food into smaller bites to make it easier to eat, otherwise you end up with mental indigestion! How you do this is up to you: do whatever you can comfortably manage to do. But obey the rules of arithmetic priority.