Estimate how long each step in your technical plan will take to implement. This will often involve research into the details (“is there already a library to do this or not?”). Depending on the nature of the project, throwing together a simple prototype might help reveal a lot of potential future pain points.
For a good guide on what sort of thinking should go into your technical plan, check out this article by Alicia Chen. One key point is to iron out any potential ambiguities with the PM or other stakeholders so that you don’t end up building the wrong thing and having to start over.
Granularity is important here. If any step feels murky or vague you’re either hand-waving (and should learn more) or you need to break it down into smaller steps. At the same time if a step is too fine-grained it might be brittle enough to invalidate the whole plan in practice.