You do not have to be a master tailor to create amazing girls' fancy dress costumes. With some creativity and ingenuity, anyone can assemble something that to please their little girl without unnecessary stress. Your fancy dress creations do not have to be perfect by any means—but if you know how to use the materials available to you, the process goes more smoothly.
Use What You Have
Chances are that you have some components of a fancy dress costume already lying about. Look through your daughter's wardrobe to see whether she already has something that might do the job. It could be a pair of denim shorts or old battered jeans that could be turned into shorts for a Pokémon Misty costume or a white blouse for Mary Poppins. Solid coloured garments, whether black, white, or another primary colour, make a great foundation for more creative costumes, such as LEGO bricks.
Repurpose Secondhand Clothing
If you are handy with a sewing machine, shop for secondhand clothing that you can repurpose and shape into other garments. Men's dress shirts can be cut and reshaped into chemises for girls' pirate costumes, while hoodies can be decorated for costumes, such as Jack Frost ("Rise of the Guardians") for Vanellope Von Schweets ("Wreck-It Ralph"). Another classic example would be to take a yellow t-shirt or shift dress and sew on some black ribbon to make a bumblebee costume.
With the foundations of a fancy dress costume already in place, break out the crafting supplies for the final touches. Use duct tape to create stripes of colour on jackets and other garments. Ribbon and lace can create additional stripes and ruffles for a more frilly interpretation. Tulle and some wide ribbon can be converted into tutus and skirts for fairy fancy dress, or even into a mermaid bottom if you bunch the tulle around your girl's calves. Use foam board cut into wing shapes with pieces layered on each other to create wings for a Pegasus fancy dress costume, such as a Fluttershy from "My Little Pony: Friendship is Magic".
Mind the Details
Pay attention to details to make more convincing girls' fancy dress costumes. No matter how you interpret the costume—whether turning Darth Vader into a princess or your daughter into a Dalek—be sure to incorporate the most recognizable elements. A whisk or egg beater is an easy substitute for a Dalek weapon. Use yarn to create wigs for characters with interesting textured or a lot of hair. This technique works well for costumes, such as Merida from "Brave", Rapunzel from "Tangled", and even Sally from "The Nightmare Before Christmas". A stencil and some fabric paint can turn a black dress into that Princess Darth Vader or a white dress into a princess R2-D2.