Using another half of a clothespin, push the top spring arm back toward the notch you modified.

Hold both pieces firmly. Not that you will die if it slips. Maybe you like minor finger abrasions. Knock yourself out. (Not shown: Oversized cartoon mallet.)

If you find that the spring arm won't stay cocked, you need to further modify the notch. You may need to make the notch a little deeper, or make the flattened edge even flatter, or both.

Here is the lower half of the gun with the spring in the cocked position. You are ready to test your trigger assembly...