If you want to dress up as Sandy from Grease, it isn't too hard. Which Sandy though? The one who is demure and sweet? Or the one who gets all sexy? You could try either look for your fancy dress occasion.
Dress as demure Sandy. Purchase a wig that is shoulder length and blond. There must be little bangs at the front. Put an Alice Band on in a pastel shade.
Wear a voluminous 50s skirt with petticoat underneath. And socks with lace-up soft shoes.
Wear twin set tops with pearls and a neat and quiet watch.
Keep the makeup low key.
Dress as sexy Sandy. This time you will need a bouffant blond wig with many, many large ringlets.
Add large round hoop earrings.
Get the clothes! A black leather jacket (polyvinyl will do but is fairly gross), black top and black leggings that go just past the knee.
Wear red high heels. They must be high and sexy.
Make the makeup shout. Bright red lipstick, lots of mascara, just cake it on and look sexy.
Go around saying either "I'm hopelessly devoted to you" or "You better shape up" to amuse your fellow party-goers.
Lastly you need pink ladies written on the back of you top
Play the soundtrack to Grease really loud in the background as you get dressed. It adds atmosphere and will help put you in the frame of mind of Sandy.