overwhelmed at work
As I see it you have several projects here:
Project 1. Describe your job for your own purposes. If possible, know explicitly what your boss wants and when, including what types of things to defer due to unexpected in-flow, when to pick these back up and what to direct elswhere. Asking the boss is not always they best way to find this out; sometimes observing is more useful for determining this or envisioning yourself. This depends on your boss and your level of employment. In a conventional "big business" setting I would refer to my written job description but if it differs greatly from what it seems is coming your way, you need to collect information about that and decide what your options are. If you don't have a job descirption, you might write one for yourself just to clarify your role and expectations. Your outcome should be a list of easy to read and remember statements describing what you are supposed to do.
Project 2. Determine what aspects of the job are hard and unmovable features and what are soft and flowing. Some jobs are 90% unexpected in-flow and about a 10% semi-permanent framework of firm and clear expectations around which the rest flows and bumps. For example, the work of a nurse in a doctor's office: the 10 % might include record all communications with patients in their charts, order supplies on Wednesdays, clean and sterilize stuff daily. This has to be done and is predictable. But the other 90% can be lots of things and they take on unknown amount of time on different days. This might be arrange for referrals, triage and follow-up, assist in procedures, educate patients on their illnesses and medications. If your job in fact has these two categories, your outcome will be two lists. Possibly the framework items are hard landscape for you and will need to be scheduled at certain times. In one of my jobs (homemaker), I am responsible for certain hard landscape items: getting kids to and from activities and school on-time, making 4 meals a day, one of which has a shifting time, for making sure all bedding and cloths are clean by Sunday, and all bills paid by the 5th and 15th of the month. Everything else is flowing. If meals get too complex, people wear a ridiculous number of cloths, or the bills get lost, this impeds the flowing items. If the flowing items suddenly change such as a kid getting sick, it impacts the framework and the fraemwork needs adjusting on a temporary basis.
Project 3. Identify barriers to completing the specific aspects of the work and then turn these into GTD projects unless you can dispense with them in one-step two-minute actions.
The suggestion that you start one-hour before everyone else is a great one if you can do it. It also makes a super good impression. Another helpful tactic when you put something through the GTD processing is to ask yourself "What will happen if I do this and what will happen if I don't"?