In many cases if the questioner were to approach their project in a logical and systematic manner then most of these questions would answer themselves
That's what professional engineers do. they work systematically through the project to achieve a satisfactory end.
This instructable contains the essential parts of one of the many student handbooks I prepared for my students for use in their exam year.
Some parts have been edited because they apply to the UK exam system and the user should insert their relevant exam criteria in there.
Please bear in mind that this was aimed at the UK school Design and Technology system BUT the methodology and information can - and should be - applied to any project that is less than trivial even work at home if of a suitable scale.
The principles can, and should be, applied in all industry small and large because they are well established good practices.
This work is all my own although the ideas are drawn from established systems in industry, exam appropriate information applies to the UK AQA exam board 2009 - This should still be current although the form is easily changed.
For Teachers and School students following this is a proven way to get good results - It still requires students to do the work.
You are more than welcome to use this information to your advantage. Please credit this instructable.