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I guess this question does not fit well to this site and may be closed. So, I post my answer before that. 

I'm actually an algorithmic engineer, not an electronic engineer. But your question is extremely general, and valid for every field of engineering. Also, it obviously depends upon the human and technical resources available to you. So, it cannot be answered at this level. 

Nevertheless, according to my point of view that every question can be answered, it has only to be answered at the same level of generality it has been asked, that is, the highest level. 

Obviously, you have to detail each task separately to manage your project, and to arrange them in a list. Then you have to evaluate each task according to your human and technical resources, knowledge etc. 
That's as simple as that apparently, but that's also all the difficulty. 

Now come the secret, I mean my secret to do that, and believe it or not, it works incredibly well.

**Rule 1:** When you evaluate the time to allow for a task, do that as well and as carefully as you can. So far so good.  Let t be the time allowed for this task.
 
**Rule 2:** Then ask yourself if you have already done some similar task, or if you have never done something similar. 
If you have already done something similar, multiply t by a factor between 1 and 2, according to the similarity of the task. 
But if you have never done something similar, multiply t by a factor of 5. Yes, you have correctly read, five time what you have evaluated. That's my secret.