The total pressure of the gas mixture is the summation of the partial pressure of all the gases in the container or Ptotal = P1 + P2 + P3 (have 3 gas species in mixture.)
The most important thing to remember when you are doing calculations for Dalton Law of Partial Pressure is that each gas volume is the total volume of the container. So if you need to determine the partial pressure, you need to have the moles of the gas, temperature of the gas (in K) and the volume of the gas (as we just stated it is the total volume.)
A common example when you use the Dalton's Law of Partial Pressure is when one collects gas "under water". That means you are collecting gas above a liquid phase (the liquid acts as a barrier not allowing the gas to go away). As you know, whenever you have a liquid phase, there is some of its liquid's vapor in the gas phase. So for collecting gas above water, there is a mixture of gases, the gas you are looking for and water vapor. So the pressure of the gas phase is actually made up of the partial pressure of the gas and the partial pressure of water vapor.
PTotal = Pwater vapor + Pgas
so if you want the pressure of the gas you must use the equation,
Pgas = PTotal - Pwater vapor
Example: What is total pressure of a mixture of gas that contains oxygen gas and nitrogen gas. The partial pressure of the oxygen gas and nitrogen gas was 278mmHg and 541mmHg?
PO2 = 278mmHg
PN2 = 541mmHg
PTotal = PO2 + PN2 = 278mmHg + 541mmHg = 819mmHg