Maybe the worst scratch is from chain. it can be so big that you cant cover it. On my old bike its soo ugly that i am thinking of sanding and painting frame . But on new bicycle i dont have that scratch. Why? because i am using chainstay protector. Not fancy or expensiv. Its just an old cloth ( i used it from old jeans) wraped around chainstay and secured with zip ties . Its so simple and efective .
Almost forgot. You dont only protect chainstay from scratches, you also protect it from mud,oil,...
So from materials you will need some old cloth ( piece of jeans,tshirt,etc.)
How to make it ? Take that cloth and wrap it around the chainstay ( if you are using thin cloth wrap it few times) and then take zip ties and secure it.
OH btw you can use diffrent materials. You can use electrical tape ( i saw few guys use it , but i dont like it. Electrical tape leaves sticky mess and also you will need a lots of tape )
You can use old inner tube, just wraped it around (also good method, a lot of people use it)