For example, if your normal landing speed is 50 knots CAS, and you have a 10 knot tailwind, and you also touchdown 10 knots too fast, that is, at 60 knots CAS, you will almost double you landing roll-out distance, if all other factors are equal. If all that sounds too complicated, just don't land downwind. They aren't intended to substitute for manufacturer's information. Consult your Pilot's Operating Handbook or Owner's Manual for specifics. One recommended technique many pilots use is to divide the gust factor by two and add this to the normal approach speed.
Some pilots add all of the steady wind and half the gust, or all of the gust and no steady wind.
To increase safety, your final approach airspeed needs to be precisely calculated, then precisely flown. But don't forget-your approach airspeed and whatever gust factor you select to add to your final approach airspeed should be flown only after all maneuvering has been completed and the aircraft has been lined up on the final approach.