Why is there 弄了 in the sentence 婶婶找来几片叶子，给雪弄了一双耳环?
I assume the object in your question is actually a snowman 雪人 not just 雪. So I think it should be "婶婶找来几片叶子，给雪人弄了一双耳环".

So we have in English: "Shen Shen found some leaves and made a pair of earrings for the snowman."

If you check the dictionary meaning of 弄 you will see it is quite similar to the English make. In the Chinese sentence you have 弄了 so that changes "make" into "made".

Without this, if the sentence was "婶婶找来几片叶子，给雪人一双耳环" the sentence would be: "Shen Shen found some leaves and gave them to the snowman." Now this could technically still be correct based on the context of what's going on, maybe it's a magical snowman who is getting earrings as a gift, but you should be able to see how removing 弄了 changes the meaning of the sentence completely.

"婶婶找来几片叶子，给雪人*弄了*一双耳环"

"Shen Shen found some leaves and made a pair of earrings for the snowman."