Answer:
Depends on your definition of 'collective noun.' If you mean a plural noun, the most common way is to ad an S to the end. A true collective noun, however, is unique to each situation and you just have to look it up.
You don't turn a noun into a collective noun.
A collective noun is a word for a group of something for example herd is a collective noun. We say a herd of cows, a herd of buffalo.
Another example is flock. We say a flock of birds a flock of sheep.
Others: a band of men, an assembly of toys, a bouquet of flowers, a bunch of grapes