The Big Mac PPP exchange rate between two countries is obtained by dividing the cost of a Big Mac in one country (in its currency) by the cost of a Big Mac in another country (in its currency). This value is then compared with the actual exchange rate; if it is lower, then the first currency is under-valued (according to PPP theory) compared with the second, and conversely, if it is higher, then the first currency is over-valued.
For example, suppose a Big Mac costs £2.00 in the United Kingdom and $2.50 in the United States; thus, the PPP rate is 2.00/2.50 = 0.8. If, in fact, the dollar buys £0.55, then the pound is over-valued with the respect to the dollar.