Cut the journal cover material and the pages to the correct size for your journal. Remember that the pages need to be twice as wide as the desired width of an individual page. Make a jig, or hole-punching guide, out of a piece of paper that is the exact same height or length as the journal (the width is unimportant). Draw a line lengthwise on the jig, about 1/2 inch from the edge of the paper. Draw four evenly spaced dots along the line, making sure that the top- and bottom-most dots are at least 1 inch from the top and bottom edges of the paper.
Lay the pages in front of you as if they were sitting in the open journal. Fold and crease each page down the middle, taking care not to skew or overlap any pages. Create signatures (the page bundles that will be bound) by creating small stacks of four or five folded pages each.
Attach the jig you made to the front cover using paper clamps, aligning the edge of the jig with the left edge of the cover so that the dots are 1/2 inch from the cover's border. Through the jig, mark the cover at each of the four dots. Remove the jig, and punch holes through the cover at the marked spots. Repeat this process for the back cover, this time aligning the dots with the right edge of the cover. Use paper clamps to attach the jig to one of your signatures (the small stacks of paper); this time, align the drawn line on the jig with the inside of the signature's crease. Use the jig as a guide to mark the crease, and use a needle or pin to punch holes through the signature. Repeat this process for each signature.
Measure out a length of string or thread; the length of thread needed is equal to or greater than the value obtained if you multiply the length of the journal's spine by the number of signatures, then add an extra two spine lengths to account for the covers. Thread the needle, then tie a knot in the far end of the thread to avoid it pulling through the holes. Place the front cover facedown with the holes on the right side, then place the first signature on top of it so that it is opening towards you.
Poke the needle and thread through the top hole in the first signature's crease, and through the top hole of the inside of the front cover. Wrap the thread around the spine side of the front cover, bringing the needle in between the cover and the signature. Cross the thread and needle underneath the first thread, creating a sort of chain stitch, then poke the needle back through the top hole of the first signature from the outside of the crease. If done correctly, the needle should now be back where it started, on the inside of the first signature, which should now be partially stitched to the cover. Pull the slack string through the hole, and gently pull on the thread until the stitch is snug. Repeat this process for the second hole in the signature and cover, running the thread underneath the first stitch before poking it back through the signature. Do this again for the bottom two holes, each time running the thread underneath the stitch before it. Make sure everything is snug; you have now stitched the first signature.
Fold the first signature closed against the cover, and place the second signature on top, opened towards you. Repeat Step 5 for the second signature, except this time starting at the bottom hole and stitching your way to the top. Each time, the needle should be poked from the inside of the top signature through all of the signatures and the front cover, but then looped around the side of the cover and behind the stitch between the cover and the first signature. When poking back through, do not thread through any signature but the one being attached. Repeat this process for each successive signature until you have added enough pages. Make sure to pull the thread snug after making each new stitch.
Repeat the stitching process one last time to secure the back cover to the journal. Tie off the end of the thread, secure it to a button, or run it around and tie it to the other end of the thread in the first signature. Your journal is now bound with coptic stitching; feel free to decorate the covers as you please and fill the pages with writing or drawing.