United entertain Smith's Rangers this week.
Fletcher told the Scottish Sun: "I had six months with Walter when he was assistant manager with us at United.
"I was actually talking to the Rangers lads about it when we were with Scotland last week. Walter came down and took the coaching sessions - setting up the cones and stuff like that.
"It's things he doesn't do now, he oversees things more these days, but when he was assistant at United his training sessions were great.
"The only thing was his rule was you don't get fouls in training - he never awarded free-kicks. It was just a kicking match at training.
"The foreign players didn't like it, but all the British lads absolutely loved it!"