How to write a basic roast!
Writing bits to roast other players might take longer than getting a one-liner ready, but bringing an original performance to a room and making it topical will probably be worth the effort.
Look for a regular player in the room and find any pattern in their character and their actions, and tie those two things together. For example:
A player looked like Donald Trump with black skin, and I noticed that every time he took the stage he just moved his eyebrows wildly. He ended up being popular with this act and was referred to as Black Trump, so I tied his lack of speech with his appearance as Trump:
"Black Trump is a lot like white Trump. Both haven't said anything intelligent so far..."
Spending a second to build anticipation for a roast might help capture the audiences attention. So I simply add "I've got another roast" when going back up to the stage.
So another example would be - player1 was repeating a joke about ground beef, so I went for a joke about saying the same thing over and over again:
"I've got another roast. If player1 was a pokemon they'd be ground beef, because that's all they can say..."
You could also add a short closer, because if your bit is any good then a closer can be satisfying. So a final example - player2 was using a long-faced horse as a running theme for their jokes. I tried to work the theme into my roast:
"I've got another roast. Player2 saw a dead horse and started beating it, because it had a long face. Please clap..."
My delivery is pretty bad, so I went for the cringey Jeb Bush closer of "please clap." But people would actually clap and tell me to get back on stage, so I don't think they mind if the jokes are bad, maybe as long as they're original.
If you get nervous telling a joke like I do, then I'd write the joke out so I can't forget it, and pace myself to make sure I don't tell it too fast or quietly. I make myself pause between the opener and the roast line to not lose that moment of anticipation, and pause again between the roast and another roast(or the closer) to allow for an audience reaction(if any).