if(conditionOne()){

//  Write 100 lines of code here
  if(conditionTwo()){
// add 100 more lines of code
  } else {
// add 100 more lines of code
    if(conditionThree()) {
     [...]
    } else {
     [...]
    }
  }
} else {

[...]

}

And prefer either achieving the same effect with methods instead of the long code blocks, or where possible using a switch statement for clarity:

if(conditionOne()){

  methodToDoConditionOneWork();

  if(conditionTwo()){
    methodForConditionTwo();
  } else {
    methodForConditionTwoB();
    if(conditionThree()) {
     [...]
    } else {
     [...]
    }
  }
} else {

[...]

}