I'm just going to copy/paste if off the cheat page I found:
Enter the following data into the console (CTRL + SHIFT + C) and fill out the code with their respected skills.
Using the console you can enter the codes below for this cheat, this requires "Testing Cheats". This will automatically set the skill level of your current active Sim. Make sure to replace the with the available options below followed by the level you wish to set it to. stats.set_skill_level Example: stats.set_skill_level AdultMajor_Logic 8 Result: Will set your Logic skill to Level 8.
Effect Code
Selects the Baking Skill AdultMajor_Baking
Selects the Charisma Skill AdultMajor_Charisma
Selects the Child's Creativity Skill Skill_Child_Creativity
Selects the Child's Mental Skill Skill_Child_Mental
Selects the Child's Motor Skill Skill_Child_Motor
Selects the Child's Social Skill Skill_Child_Social
Selects the Comedy Skill AdultMajor_Comedy
Selects the Cooking Skill AdultMajor_HomestyleCooking
Selects the Fishing Skill AdultMajor_Fishing
Selects the Fitness Skill Skill_Fitness
Selects the Gardening Skill AdultMajor_Gardening
Selects the Gourmet Cooking Skill AdultMajor_GourmetCooking
Selects the Guitar Skill AdultMajor_Guitar
Selects the Handiness Skill AdultMajor_Handiness
Selects the Herbalism Skill AdultMajor_Herbalism
Selects the Logic Skill AdultMajor_Logic
Selects the Mischief Skill AdultMajor_Mischief
Selects the Mixology Skill AdultMajor_Bartending
Selects the Painting Skill AdultMajor_Painting
Selects the Photography Skill AdultMajor_Photography
Selects the Piano Skill AdultMajor_Piano
Selects the Programming Skill AdultMajor_Programming
Selects the Reaping Skill AdultMajor_Reaping
Selects the Rocket Science Skill AdultMajor_RocketScience
Selects the Video Gaming Skill AdultMajor_VideoGaming
Selects the Violin Skill AdultMajor_Violin
Selects the Writing Skill AdultMajor_Writing