How do I re-wire a 4 wire plug to a 3 wire plug for a 220v washer/dryer?

The code is a 4 wire receptacle.I believe this is to insure a good ground.The old 3 wire did not do that. I work in the apartment maintenance field. We run into this when someone with an older dryer moves into a new apt.There are directions on a new 4 wire cord to wire it up.I believe the red and black supply the 220 the white is a common for 110 use on the dryer,such as the control knobs and or a light.The green bolts to the frame for a positive ground. We insist they buy the 4 wire cord and we will install it.They always try to give me their old three wire and I tell them to keep it as it is 50/50 chance when they move,it will be the 3 wire again.Please check when you buy the cord, the hardware store will confirm what I've just told you.ADDED INFO The other ans are correct saying be care full. As stated there is a 4 wire cord and plug made for this purpose the new type receptacle is to protect you. This should be wired correctly.

That's a good question. I'm gonna try. 240 v needs only 2 wires + ground. Left and right from power, no neutral. Third wire should be capped off. Which one that is I can't say for sure. 240 will run from both plates in the fusebox/circuit breaker/opposite "fazes" there will be no neutral. 110 has a neutral contact. Does that make sense? You may need to replace the breaker as well, I am not sure.