I looked on Youtube and found that these were the best two make up tutorials, they're both ever so slightly different, using different types of make up for some aspects depending on what you have already available so to not spend too much :)
vintageortacky on Youtube also has a good four part tutorial, which isn't as accurate but you may prefer!
Here is the generic Sally dress, you can buy all different sizes on Ebay and Amazon, it is exactly like the movie dress, but a little bit pricey!
If you want something a little bit different from the regular dress but still want to look like Sally this is a good thing to try!
You can also buy 'Sexy Sally' costumes, but I'm not encouraging that, I really can't image movie Sally the Ragdoll showing off her bits.
Here is explained how to make your own Sally dress and wig :)
If you're really going all out, get your boyfriend or a friend who is a boy to be Jack Skellington!