After you braid all the way across your head continue braiding about three more inches or even to the end if you want, that way the braid can be secured in the elastic when you put the rest of your hair in a bun. I use a thin clear plastic band so it wont be seen. The next step is to tease the crap out of your crown and on the sides so that when you gather your hair in a bun you have some volume. For this style you really need some volume on the crown because the inverted braid does stick up from your head a little and if it is taller than the rest of your hair it look a little crazy. After you tease gather all your hair in a bun on the side of your head. You need to put the bun on the same side that your braid ended to cover up the elastic. The bun can be really free form because you are going to pin it how you like. The way I shape my buns (hehe) is to loosen loops from the elastic, twist them, and then pin them where I want them. this creates a really messy look, which I like, but also allows you to have a lot of control about where the bun is on your head, how far it sticks out, and the circumference of the bun.
When you I have my bun looking like I want it, I always add a little glam to it by pinning in a flower. The one I am wearing is from Forever 21 and is featured in one of my old blog posts. You could also pin in antique brooches, that is another on of my favorite ways to add a little more to this style.