According to the Hanafi school missed prayers are made up in the order in which they were missed (observing tartib). That is, the earliest missed prayer first, then the one after it.

When tartib no longer applies
Once the number of missed prayers passes six, a person is no longer “sahib at-tartib” and the obligation to keep the order falls away. In that case they may be made up in any order.
Making it easier in practice
Someone with few missed prayers performs the make-up prayer before the fard of the current time.
Someone with many can add one missed prayer of the same kind alongside each obligatory prayer.
A little at a time but regularly is far easier than pausing and letting them pile up.