I've been fielding a number of questions about riding in a skirt, so I thought I would provide a few tips.
In general, cycling in a skirt is no different from wearing anything else. Sure, if you're planning to ride more than 20 miles at one go, I would change into some cycling-specific clothes with padding and no oddly placed seams. But if you are going 5 or 10 miles around town, it's all about the same. It just seems unfamiliar.
Some outfits may give you trouble on a bike. But this is no different from finding that your new skirt bunches up when you walk down the hallway, or gets twisted, or tends to snag on furniture. The only thing that really doesn't work is a tightly-fitted pencil skirt, unless it has a major slit (preferably in the back) or a kick pleat. This black shirt is fairly straight, but the material stretches and there is a 4 inch slit in the back.
If you have a very long, full skirt, there is a chance that it could get caught in your brakes, chain, or wheel, which could cause a crash. A full chain case and skirt guard will prevent this, and may be a good investment if you wear long skirts a lot. If you just need a one-time solution, you can simply gather up part of the hem and secure it with a know, a rubber band, or a hair clip. I prefer clips because they look nice and don't damage fabrics. They can also secure gauzy fabrics that fly around too much.
Having a step through frame makes it more brainless to ride in a skirt, but it's entirely doable on a diamond frame. You just need to keep your knees together as you mount the bicycle, and to have enough fabric to go over the top tube when your feet are on the ground or extended. It's easy!