How to Bike in a Skirt Without Wearing Shorts Underneath?
First of all, you don't need to ride a step-through to bike in a skirt. It's never made any sense to me how the presence of a top-tube could mess with my outfit while riding, but it does make the mount/dismount a tiny bit easier. I don't have a step-through so when I'm getting on I just tilt my bike sideways until I can get a leg over without flashing anyone.
Skirt length is really important if you opt to go shorts-less. Make sure you wear something that hits at just above the knee or longer. Cycling posture tends to lengthen the back of your torso making a mini even shorter.
The weight of your fabric is really important, too. Anything gauzy or flow-y tends to blow back off your thighs in your self-generated breeze. Look for sturdier twill weaves that have a bit of heft.