1) Find out what is the Operating Profit ( ensure that they have consistent Operating Profits

2) Divide that by an appropriate interest rate, say 4%
3) That would give you the Present Value of Future Operating Profits ( a Perpetuity ). This is how much the business is worth
4) Divide that by the number of shares
5) That would give you the "fair value" per share
6) Compare that Fair Value with the current share price, to have a feel of your Margin of Safety
Of course, one can put in various scenarios for the Operating Profit ie. Pessimistic, Realistic, Optimistic
And one can also discount things at various interest rates eg. 4%, 6%, 8% and 10%