Focus on EVERY single step, as trivial as it may seem. Many people, when it comes to calculations etc, go into 'automatic gear' or 'cruise control', and just mindlessly do the calculations without thought because it is so easy.
Kind of like doing something repetitively, you start doing it unconsciously because you are so used to it.
However, it is BECAUSE you are doing it unconsciously, that silly errors arise.
So concentrate on every single thing, regardless of how basic it may seem.