An accomplishment journal, win diary or whatever title you want to put on it all means the same thing - somewhere you write down all your accomplishments through your career. It’s one of those high leverage things which is low effort but provides high reward once you stick at it. Tracking achievements is useful early in your career too as you move up from the entry level position.
There’s a bunch of places where having a quick and easy list of your achievements can come in handy. For example when you’re brushing up your resume while looking for a new job or an internal promotion, you want to be able to focus on telling the story rather than trying to remember what you did a few months ago. It’s even just useful to glance over while preparing for a job interview to remind you of some little details and keep you focused on what you’ve done. It can also come in handy when you’re in a rough point feeling like you’re making little or no progress in your day to day - being able to look back and see all the things you’ve accomplished can be a powerful motivational factor.
There’s not really any requirements to keeping record but keep it simple. There’s two pieces of advice I would recommend following. First, it has to be somewhere that you take with as you move across jobs or devices. You don’t want to have to start fresh if something happens your computer. Second, it has to be something easy to do. If it requires too much thinking or time then it’s going to be something that’s going to get cut at the first sign of trouble. My advice is to keep it as a simple note (markdown or your favourite notes app) and then just append to that.
The entries themselves should be descriptive enough to include what your responsibilities were but not too long as to take ages to write and a burden to scroll through. Remember, these are for you to read so you don’t need to include all the little details, you can add those yourself later on if you’re using the entry somewhere else.
Having said that, ideally the format is close to the resume format where there’s a couple of sentences summarising what was done, what was the impact and how you contributed to the outcome. It is also a good idea to include the date and your job title for future reference. The main goal should be to focus on your impact as that’s the whole idea of the document.
And that’s it for keeping track of your accomplishments. For something that’s easy to keep doing it can have outsized rewards over time.