Difficulty: Moderately Easy
Step1
The best scenario is if you know where the squirrel is getting into the building. This helps to give you a focus for where to set the traps and/or poison. If you don't know, guess and set the traps around these areas. Set the traps along walls. Squirrels travel along walls for security.
Step2
Bait. The best bait for any of these traps is peanut butter or apples. Squirrels hunt with their noses as much as they do with their eyes. Wire the bait onto the trap so it can't carry away the bait without springing the trap.
Step3
Buy a live trap. This is a lightweight wire trap that can be baited. If you do catch something, be safe and use thick gloves when removing the trap so you're not scratched or bitten. The issue with catching squirrels live is that they are very territorial. If you let it go in the area you caught it, it will return to where it was. If you take it several miles away and let it go, it may starve because other squirrels have likely claimed this area and a new one won't be welcome.
Step4
Leave out poison. "Bar Bait" is a brand of rat poison that works well. "D-Con" is another that works similarly. The theory is that when the squirrel eats the poison, it becomes very thirsty and will leave its dwelling to find water. While its looking for water, it dies somewhere near its dwelling where you can see it and dispose of it.
Step5
Set traps. Two kinds of traps are rat traps and glue traps. Rat traps are simply large mouse traps. They are on a wood base with a metal spring. They are very inexpensive and if a squirrel is caught, you simply dispose of the entire trap. Glue traps are simply plastic trays full of glue. The package says they are pre-baited, but putting an extra dab of peanut butter on the tray never hurts. The squirrel goes for the peanut butter while stepping in the glue and becomes trapped.
Novice 
