Observe her ease. Janet is either comfortable in her own skin, or is really talented at faking it. Do what you can to be comfortable with who you are. Wear clothes that make you feel pretty, and do a warm-up that helps you stay focused and in your body. When it's time to move like Janet, you want to appear to feel as Janet does. She's in control.
Copy some of her signature moves. Blast her retro albums and reminisce. You can probably still do her moves, or simply check out her videos if you stumble. You want to rely more on your imagination, so that you throw in your own creativity, but it's perfectly fine to refer to her old videos and concert footage. It will lead you in the perfect direction for copping her moves. There's no disputing footage and visual evidence.
Enlist the help of a sibling, friend or other relative. Have someone watch you as you move. While this can be embarrassing, an observer can watch both you and Janet videos simultaneously. He can tell you if you are missing something. A second opinion is sometimes all you need for definite improvement.
Make a list of all the things in Janet's videos that you like the most. Decipher what exactly about her moves gets to you and makes you want to do them, too. Look over the list. Pull out the moves that you want to put into your own routine.
Put on a pop song. It could be a Janet Jackson song, or it could be any other song of your choice. You are going to pull from the list you made in Step 4. Do all of your favorite moves together, in sync to the song. You have reinvented the dance moves of Janet Jackson, but now in your own way. You can do her best moves in your favorite order. Congrats, you are now dancing like Janet!
Practice your Janet Jackson moves. Dancing, like any activity, takes a lot of trying and practice. Commit to it as you would any other undertaking.