well... one of the ways. There are many ways to cover yourself but this is how I usually do.
Under the abaya, you can absolutely wear anything as long as people outside cannot see them. If you happen to wear an open front abaya, just be careful with the clothe you choose to wear under it. If you wear tights, skinny jeans, leggings, sheer dress or short skirts, people may see it because when you walk, the abaya will surely close and open naturally.