Measure your book around the spine from the edge of the front cover to the edge of the back cover. That is your width.
If you want to keep things super simple you can of course cut one piece of fabric for your outer cover and just quilt it down to your batting.
Take the two pieces you cut as described below and fold them in half wrong sides to wrong sides. Place the raw edge of that folded piece of fabric along the side edges of each end of your journal cover.