Work on achieving a British accent. One of the most distinguishable things about Simon is his British accent. Whether he is delivering insults or compliments it would not be the same if it were not delivered with a British accent.
Learn the history of music. Behind the scenes, Simon is a producer of some of the top selling acts in the music industry and he has a good ear for what will be successful. By knowing the history of music and what will be more successful, you can judge singers more successfully.
Cut loose! Do not hold back when you are judging. One of the most entertaining parts of American Idol is when Simon bashes on contestants, and he does not hold anything back. If he thinks something is horrible he will tell you.
Work on as many insults as you can. You do not want to get repetitive so keep them fresh and interesting. Some memorable ones that Simon has used are “That was like a hamburger with only the buns” and “That sounded like an amusement park act”.
Judge more than just the singing. American Idol is known as a singing competition, but Simon also likes to comment on other things such as their style, size, and even the way people talk. Focus on these aspects as well and judge them as critically as possible.
Emote. Use facial expressions. Instead of using words, Simon can express his opinion with just a facial expression. Bugging out his eyes, putting a smile on his face as well as other mannerisms have been used instead of words and they get the message across loud and clear.
Study past episodes of American Idol. You can watch and pick up on things that Simon does and apply them to your own singing or judging.