Bear with my horrible instructions. This is how I have done it in the past...
Open the journal you want to cover and lay open with the spine and both covers flat against the fabric you want on the outside.
Trace around the outside of your journal. Then cut the rectangle out, making sure to leave about 1/2 inch border for your seam.
For the fabric on the inside (the felt), trace just the cover twice and cut out shape, leaving a seam allowance on the two short sides and one long side.
Sew the inside and outside pieces along the dotted line as shown, making sure the right sides of the fabric are together. You can hand sew or use a machine.
Trim the excess fabric around the seams down and then flip inside out. It should now slide snugly around your journal. You can either trim the weird little extra pieces around the spine down or flip them over and sew them.
I hope this isn't too confusing and at least sort of what you were looking for!