I just went through doing this in 2019 - 2020 in a more structured way. Prior to that I was haphazardly trying to play out my life since 2014 to limited success. Here is what I did:

1. Buy a notebook. Get a pen and put it to paper. YMMV, but I've found using apps for any kind of deep thought is distracting. It only takes one notification to put you off track. Also I've found writing things has the effect of making it more "real". Use this notebook to keep notes and adjustments to your plans. Don't fall into the trap of keeping it "pretty" like that one subreddit that worships perfectly written out notes. Life is messy, expect this notebook to be so too.

2. Figure out your ultimate goal in life. You should be able to summarize it in once sentence, maybe even just a few words. This may take you some time. It took me years to get down to my ultimate goal: "Age gracefully". Write it down in your notebook. You may go through multiple revisions of this goal. Maybe you start off with a 2 page manifesto, then break it down to 2 paragraphs. Point is you ultimately need to get it down to one sentence.

3. Figure out what obtaining your ultimate life goal entails. For me aging gracefully means having Financial Independence, Being Healthy, Building a Tribe and working on a Passion Project. If you write down your ultimate goal in the center of a page, each of these other sub items should be a circle surrounding your ultimate goal.

4. Write down in detail using numbered bullet points what gaining each of the sub items means. For example Financial independence for me had 4 sub items: Not being in debt, Growing asset portfolio, passive income and adopting a anti-consumerist mindset. Make as many sub bullet points under each of these as you need. Start going into micro details in your sub bullet points. Make plans for how you are going to get yourself to do these bullet points or achieve them.

The big picture goal here is to start with what you want at the end and work backwards by one step thinking of what you need to have done just before goal x to get there.

From there keep coming back to your written goals. Make notes, make updates. Make it a conversation back and forth with yourself. Maybe something didn't need to be a goal, maybe you need more goals. My notebook is full of notes, scratched out things and updates. Seeing my thought process on paper gives me the clarity to keep making the right plans as I move forward in life.

As for timelines, I generally try to set timelines for what I want in:
- This week (short term TODO items and errands)
- Monthly (Bigger picture theme I'd like to accomplish for the month)
- Quarterly (Each month's goals should contribute to my quarterly goals)
- Yearly ( 3 - 4 things that my quarterly goals feed into)
- 3 year outlook (Direction I want my life to be heading into in 3 years. Vague and open ended, Yearly goals sent more detail)
- 5 year outlook (same as 3 year, just another time notch to keep me focused)
- 10 year outlook( see above...even loftier, more grand and visionary)
- Ultimate life goal (should be the 1 sentence goal the guides your life)

I'm still figuring it out as I go, but if I could recommend you do just one thing it would be to get a notebook and keep writing in it to plan out your life. There is nothing quite like thinking on paper.