This article talks about how to varnish an oil painting. For why you should do it see 5 Reasons Why You Need To Varnish Your Oil Paintings.
How to varnish an oil painting
Varnish protects your painting from dirt and dust and also evens out the painting’s final appearance, making it all equally glossy or matt.
Here is a good video on the process of how to varnish an oil painting from the Winsor & Newton company.