Play with proportion.
Try a loose piece with something more fitted, or repeat a relaxed shape throughout the outfit. Look at the silhouette from head to toe before focusing on small details.
Find the connecting detail.
Repeat a colour from the outfit in a shoe or bag. A hairstyle or one strong accessory can finish the look without making every item shout. Use the in-game wardrobe to test a few versions of the same idea.
Take the idea into a Glam Battle
Open the theme in Glamour Girls and read its current brief and wardrobe requirements. Check the entry shown, style before the clock runs out, and take your finished look to the runway.
The Owambe style picker can help with a colour story. For more occasions, browse the style guides, or experiment freely in Dress Up.