Generally, if you are wearing a skirt make sure its knee length. Your skirt and blazer (yes a blazer - not a cardigan. Cardigans are business casual.) should match - i.e. greys with greys etc. Flats are always acceptable...if you don't know how to walk in heels now is not the time to learn. Nobody will bat an eye if you are in flats but if you wear heels and can't walk in them it will make quite the impression when you stumble in. In terms of jewelry, you shouldn't wear more than one of each piece - i.e only one ring, only one bracelet, and you would want to make sure you are matching your metals. Gold with gold, silver with silver. Don't make the mistake of wearing something that you'll want to play with - I can't wear bracelets or rings because I will take them on and off. Iron everything beforehand. If you are freaked out about your hair a bun is always viewed as professional and acceptable, but for vet school interviews its common for people to wear their hair down.
source - it was my job for over a year to tag people who weren't dressed in business professional