I think "Getting Things Done" is genuinely a pretty good method. David Allen wrote a book on it, and there are whole communities where people take it to a weird level. I tend to think all of that is overkill, and I'm happy to offer my takeaway from the GTD method, with the caveat that I took the pieces that worked for me and didn't incorporate things that did not:
First, try to break down big projects into smaller tasks. In order to make progress on the thing you want to do, what is the next action you need to take? For example, I want to take a trip to visit my family that lives far away. My next action is not "plan a trip." My next action might be "buy plane tickets for X date." Oh, I don't know what dates should be my departing and return flights? Then my next action is actually "determine dates of my trip." The idea of "Next Action" is important in GTD. You don't have to do the "Next Action" right away, but you need to be able to note it down.
Next, create a small list of your big projects/goals you're working towards. This just helps clarify what is important for you to be working toward. The list is personal to you and you an always update it as needed.
The real trick now is capturing all these Next Actions. Create a centralized place where you can collect your "Next Actions." For example, you might have a basket on your desk to collect sticky notes or index cards with your Next Actions written on them. But this is HN, so probably a better method is some kind of to-do list app. The important thing is that it is easy for you to add Next Actions into this bucket whenever you need to, and also that you "trust" it to contain everything you need to do next. The goal is to have everything in one place. If one "place" is not possible, try to have as few as possible, like maybe one place for your work tasks and other place for your personal tasks, or an app for most stuff and a basket for physical stuff (like mail or receipts).
Now, whenever you have time, you check your system and work through your Next Actions. Ideally you will have set aside some time to work on this stuff, but life is busy. I used to tag some tasks as "phone" tasks, meaning that X task could be done just on my phone. This was handy if I was, I dunno, stuck in a long line somewhere with nothing better to do, I could look at my "phone tasks" and check something off the list.
Additionally, sometimes you need to be reminded of things at specific times. GTD recommends having a calendar for this stuff, which is fine. A Google calendar that you can sync across your work computer and smartphone works pretty well, but I find that most to-do apps work pretty well for scheduling reminders. The important thing is to, again, have a system you "trust." In this case, you trust your system to remind you of whatever thing you need to do at the appropriate time. Tip: sometimes this means I need to be reminded a little before the actual event (like if I need travel somewhere), so build that in as needed.
Finally, GTD says you should have some kind regular review period. Once a week (or whenever works for you), you look over your whole system and clean things up. Like maybe something has changed, and what seemed important at the time is now something you can forget about, so remove those Next Actions (or a whole project) from your system. This review period should usually only take a few minutes, don't stress over it.
Ultimately, the goal here is to create a system that you trust enough to hold your to-do list for you, so you're not carrying this mental-load of fuzzy tasks in the back of your mind all the time. You put a task into the system, and then you will get to the task when it is appropriate to do so.
There's a whole book, of course. But my entry to it was through the Todoist app, and they have a pretty good summary of GTD on their blog: https://todoist.com/productivity-methods/getting-things-done