Having said that the best way to use the previous years’ papers is to randomly solve one, before actually beginning your preparation. That way you will be able to understand your weak areas better.
Then you can start your prep with an extra emphasis on your weak points. After having done so, you can go on to solve another previous year paper to weed out further the weak spots in your prep and work on eliminating them.
Carry on with this process till you are confident that there aren’t any more weak spots left in your preparation.
Lastly, if you are lucky, you might just chance upon certain questions in your actual UGC NET exam, which you had already solved before in the previous year question papers.
In the blog linked below, we have provided past year papers with a detailed solution.
You can go through the Past Year Question Papers of NTA UGC NET (2009 – 2020).