Gild the lily
Definition 1 of 1
Pro Tip 1/3Metaphorical Use
It's used metaphorically to describe overdoing or excessive enhancement of something already sufficient or perfect. 
Printing her award on golden paper would be gilding the lily.
Pro Tip 2/3Negative Nuance
The phrase usually suggests that the excess is somewhat negative or unnecessary. 
He was already praised by everyone; the speech in his honor gilded the lily.
Pro Tip 3/3Cautious Usage
Avoid using it to describe situations where improvements or enhancements are genuinely needed. 
Instead of helping, his extra explanations gilded the lily and caused more confusion.