The first thing is the MVP. As fewer distractions from it as possible. So no CSS, no JS, nothing fancy. Just bare-bones features.
Only after that, I can roughly estimate everything else.
I use Notion to better figure out how everything should look and what kind of layout I will use. Also, I use Milanote to dive deep into breaking features into small tasks.
I usually have a Brief column where I extract and expose business logic. From that column, I create an Investigation column where I double-check if everything so there are no overlapping issues or blockers. That lastly leads me to the Implementation column where I outline everything that needs to be done.
From there it is "Just Do IT" (capital IT cause we are in the IT industry) :D