It may sound silly, but it works. Get PHYSICAL! All you need is access to technology to project sentences on to white board and markers in lots of colors.
Have ready to project a list of sentences requiring commas for different reasons. As you review the comma rules, invite students assign a different color marker to represent the different reasons for using commas. You could even ask the color chooser to explain why one color seems better for one comma use than another. (Just another way to repeat the rules and understand how they work.) Here’s a link to a website that may get you started. Rules for Comma Usage. Adjust sentences for the students you have.
Then, project one sentence at a time. Give students 13 seconds to figure out the where and why for the comma(s). Randomly choose a student to select the correct color marker and place the commas on the the projected sentence, explaining the reason(s). For example, GREEN for commas in a series. Student, “I’m placing a green comma between ‘getting a visa to visit America, staying to earn my college degree, and deciding to apply for citizenship when my roommates’ family offered a me a job …” because a comma is used to separate words or phrases in a sentence.”
Then, right away, on the count of three, ask all the students to give a thumbs up or thumbs down to indicate whether student at the board has used the right color, stated the correct reason, and placed the commas in the correct places.
The deciding quickly, seeing and hearing a classmate get up, write, and explain, reinforces the rules. Making a public commitment of thumbs up/down lets students know, for themselves whether or not they know the rules.
You could make up silly sentences or ones that include names and public information about the students in the class to keep things light while making certain students are learning the rules. Perhaps, do just ten sentences each day for a week, weight correct comma use very, very heavily on the next writing assignment, then continue to include that correct use the rest of the course, but weighting the same as other issues of mechanics.