Summarize this article in one sentence.
A "mock-up" is basically a sketch of the entire comic book, page by page. They don't have to be detailed as the bigger issues layout. Instead, figure out how many frames or lines of dialogue fit on each page, where do you want any "special pages" (like full-page frames), and will the format of each page be identical or change depending on mood? This is where you start merging the words to the pictures—so have some fun.  If you're not artistically inclined, you don't need to worry about hiring an artist just yet. Instead, just focus on the basics. Even stick figures can get the point across and help your visualize the final book. While this is "only" a mock-up, you still want to take it seriously. This will be your blueprint for the final project, so treat it like a sketch for a painting and not some throwaway practice run. one for what should be shown to the reader in the story, what action needs to occur, where character development will go, etc. Other timelines will need to be made for each character, so you know what their life has been so far, where it is going, etc. These will help you keep the pages and stories straight, visualizing where each character needs to be at each portion of the book. Keep in mind pacing, so if your main character has just discovered the bones of a monster in her backyard, the reader gets to have a nice big picture to look at and take their time viewing. Remember that dialogue is actually seen in a comic book, so it literally needs to fit in each box. Try not to jam too much at once.  That said, some comic books choose to let the dialogue balloons spill into other frames, creating a somewhat looser, chaotic feel. For longer speeches or monologues, consider connecting the speech bubbles from frame to frame. The same person is giving the same speech, just with different action underneath. Many professionals will use two pages, one for the script and one for the pictures. Remember, the trick of comic books is your balance between words and visuals, and this is easiest to see side-by-side. You can tick off each caption and frame as you work. For example, the script might go:  [Page 1.] Spiderman is swinging down the streets when he spots 2 police cars chasing a yellow sports car. Caption1: Hmm it's strangely quiet today... Caption 2: I guess I spoke too soon! [Page 2.] Spiderman is swinging down the street and the two blank caption spaces. If you've been diligent about clean professional work, you might be able to turn the mock-up itself into the book. Otherwise, get to work on the actual thing, using your mock-up as the guide. Sketching, inking, and coloring a comic book is a serious undertaking. But it is also a ton of fun.  If you're getting an outside artists, send them the script and ask for samples. This helps you see if their visual style is right for you. Illustrating a comic book is a topic worth its own tutorial, as it is a challenging and exciting art form.
Use a mock-up to test out your ideas, style, layout and pacing without sinking too much work into the idea. Create several timelines: Divide a blank page into panels for your story. Using your timelines as a guide, fill in the panels with either descriptions or sketches of what action should be seen, and what dialogue should be heard. Keep your script page and graphic page side by side as you work. Hire an artist, or finish the work yourself, once you're happy with the mock-up.