Wonder Woman has worn a number of outfits thru the years, so there's no one correct approach to this costume. Our interpretation pays homage to the best elements of each.
Begin with the Wonder Woman t-shirt.
A wig, for those that need it.
Next, add the tiara. If you're the crafty type, this could be a homemade project.
Accent with matching power bracelets. Again, these could be fashioned at home.
You'll also need one of Wonder Woman's signature props, the Lasso of Truth. A stretch of rope spraypainted gold would work nicely.
Round out with blue hot pants - or mini-skirt. Affix with white stars if time permits.
And then arguably the most crucial element - Red Go-Go Boots!