X is for eXecution.
I know, I know... execution doesn't strictly begin with X. Well, once I've worked out how you can use a xylophone to get more done then I'll come back and update this entry; until then it's staying as execution.
"Execution" is the sort of word you only expect to encounter on the powerpoint slide of a high-powered business executive, but it's just a fancy way of talking about doing the tasks on your list.
And doing the tasks on your to-do list is the reason you started keeping them in some sort of system, isn't it? Execution, then, is just starting to put together the things that we've covered elsewhere in The A-Z.
Once you've broken down everything that needs to be put onto your list(s), and sorted the tasks based on their context and importance/urgency then you'll know what you need to do next. Then you need to stop procrastinating and focus on just doing the tasks. And if you keep going you'll get through the work and then you can relax and congratulate yourself on what you've accomplished.
See, it's easy once you've worked out your system...