I fold up both sides ... often the sides will meet right on, sometimes there is a small space and that doesn't matter.
You need to anchor the folded point. I have used die cuts, or just plain little strips of some of the cardstock like shown below.
Now you decorate the front of the bookmark. I love making glitzy bookmarks but one word of caution...If you put thick die cuts, popdotted designs,ribbons, jewels...they look great but the book will not close nicely once you put the bookmark over the page edge to mark your place. Once I used a darling little bookmark that had a cute popdotted owl with jewel eyes and after I closed the book, the jewels poked a nice hole in the pages before the bookmark. So struggle as I might to not bedazzle my creation, here is a flat little bookmark sporting a cute duckie on a nestie round that was embossed.
Now you make some. You will be delighted how quickly these are put together. I love making them from many different papers.