Capture everything in your head. Who you need to contact and about what, upcoming trips, list the projects you're working on.
Clarify what all this means. Why am I trying to juggle this information in my head when I can put it into a proper notes app or notebook (it's information, record it, reference it later). If it's a project, add more detail (actions). If it's action, you're good.
Organize it. Put things with deadlines into your calendar. Determine the context of actions (I have to do this on the way home, I have to do this in the lab at facility X on my next trip there). Put things into lists [0] that make sense. Make a shopping list. Have an agenda for your meeting with Bob. Have a plan for your next weekly sprint.
Review everything. We canceled this project, or we shipped that one. Actions that are left open are put on hold or discarded. I missed a deadline, or I finished this but forgot to mark it finished and so those are removed. This is actually dependent on this other thing that I haven't written down yet, so I write it down.
Engage. This is the purpose, this is your life (personal and professional). This is you getting things done and where most of your time should be spent.
[0] Lists is a broad concept. My grocery list is a list, I should fill it out and put it in my car or have it in my pocket notebook so it's accessible when I'm near the grocery store. My projects' plans are a list, but it may be in a MS Project file or an emacs org file, or even both. I have "Fix PR-123" in the main project calendar. But once I get to it, it's in emacs for me and looks more like "figure out this module and that module; update them; update documentation and tests; get it peer reviewed".
Is there a GTD lite or something for beginners ?