Hold CTRL+SHIFT+C and you can open the cheat console. Enable cheats by typing testingcheats on/off. From there, you can enter the following code from the console to set the desired level on a given skill:

stats.set_skill_level [skilltype] [1-10]

As an example, you might enter stats.set_skill_level Major_Fishing 7 to set your skill level on Major Fishing to 7. The list of available skill types is
as follows:

Major_Baking
Major_Bartending
Major_Charisma
Major_Comedy
Major_Fishing
Major_Gardening
Major_GourmetCooking
Major_Guitar
Major_Handiness
Major_HomestyleCooking
Major_Logic
Major_Mischief
Major_Painting
Major_Photography
Major_Piano
Major_Programming
Major_Reaping
Major_RocketScience
Major_VideoGaming
Major_Violin
Major_Writing
Skill_Child_Creativity
Skill_Child_Mental
Skill_Child_Motor
Skill_Child_Social
Skill_Fitness