What i did was make a image 146x399 its a good size but you can decide whats best for you. Crop the image or just resize it, Then add your personal touch with some brushes or abit of text but its completely up to you.
Then save it and click print! But make sure you print it twice one for each side of the bookmark. Cut out the two printed sides and glue onto a piece of cardboard or anything that is stronger then paper.
Then if you want ribbon on your bookmark, punch a hole where you want and tie the ribbon through the hole also you could get your bookmark laminated so it lasts longer but it looks fine without it.