I got the majority of this lesson from the Read Write Think website.
We started with a quick discussion of formal and informal. I used Ryan Gosling photos because I'll take any excuse to look at him. They definitely got the point that both outfits were fine, but had to be worn in the appropriate place. Then we rated sentences on a scale of 1 to 5 with 5 being the most formal. I pulled up the responses on Google Forms with the bar graphs to make sure everyone was similar in their rating and we talked about what made those examples either formal or informal. As a class we listed qualities and characteristics of both. Watched the quick Youtube video and then tried our own. The handout that I used is in the lesson linked above.
Students had to change informal to formal and vice versa. My favorite was this example.
"Man, I'm starving! I'm gonna get something to eat and then call you back." became...
This was a quick, but hopefully, effective lesson.