How do I find the default risk premium given corporate and municipal bonds? Meaning what is the equation I should use?

The formula for risk premium, sometimes referred to as default risk premium, is the return on an investment minus the return that would be earned on a risk free investment. The risk premium is the amount that an investor would like to earn for the risk involved with a particular investment. - Source (http://www.financeformulas.net/Risk-Premium.html)

I found a great PDF of Finance Homework, which includes detailed formulas and instructions to calculate Default Risk Premiums (http://julianvu.com/temp/FinanceHomework1.pdf).