There’s many different and successful ways to do it. Many RIPs will do it for you but I’ve seen that go awry when occasionally a panel somehow is misaligned or not scaled correctly, but that’s another conversation. I’ll tile an image by first creating the layout of the 4’x8’ boards in Illustrator, resulting in a layout of 4x8s that create the overall size of the finished sign...in your case 12’x16’. Then I’ll use the “offset path” feature to add a 1” outline to the 4’x8’ shapes I’ve arranged in illustrator resulting in an overall size of 12’2”x16’2”...this will also create overlaps of each of the panels. Place your 12’x16’ artwork into illustrator and center it on the 12’2”x16’2” group you just created. You will now notice that your original artwork actually needs to have a 1” bleed, resulting in a file size that’s 12’2”x16’2”. You can add that yourself or have the customer resend the artwork with the bleed added. So now you’ve ended up with artwork and vector shapes that both are 12’2”x16’2” and centered on each other in illustrator. Now all you do is use those vector shapes (each 50”x98”) to create clipping masks for each panel. To do this you will copy the artwork, create the clipping mask for panel 1, paste the artwork back in place (control+b) and create the clipping mask for panel 2 and so on. The end result is you have a bunch of tiles each with a 1” bleed that will be applied to your 4’x8’ MDO. I usually separate them into separate illustrator files mostly for file size and referencing later on if needed. Installation is best if you apply panel 1 to the MDO then line up the next blank board and line up the print for panel 2 to panel 1 and apply and so on. It’s harder to explain in this post then it is to actually do haha! Once you’ve done it a few times I could tile a 12’x16’ in less than a twenty minutes, and most of that would be waiting for files to save! There are other ways to do it in Photoshop or using grids of artboards in illustrator which I still do on occasion but I prefer the method above. It gives more control and a record of how the panels were printed in case the sign gets damaged and a panel replacement is needed. Also if it was a vehicle wrap and not applied to 4x8s then you could manipulate where the seams are by an inch or two to make line up for installation easier by moving seams in between words/letters or into other easier to align spots, or lining up with body lines/edges of the vehicle...or if it’s a wall mural to avoid an electrical outlet and so on. Lot of ways to do it but i find this way to be fast and give full control of the process, hope it helps!