Processing is the stage of the GTD workflow in which you make decisions. You get all the stuff you have collected and, one by one, decide what are you going to do with them.
For you to decide, you must first define clearly what each item is. Once you have a clear and specific statement of the desired result, the first question is whether you need to take action or not. If the answer is yes, the next thing is to decide what the next action is.
Here is where the famous Two-Minute Rule comes in: If the next action can be done in two minutes or less, do it now, although it is not an urgent or high-priority task.
It does not need to take two minutes exactly, it may take three or five. If you have to read a short text to make the following decision, read it. If you have to send an email or make a quick call, just do it.
If you do not complete the result with that action, identify the next action and process it following the same criteria: If you need less than two minutes, do it; if not, defer it or delegate it.
In short, if doing a task will take less time than processing it, organizing it in your lists and tracking it timely, it is much more efficient to do it the first time it is in your hands.
The Two-Minute Rule can greatly improve your productivity, making your projects move forward incessantly and preventing many small things get to overload your system.