A packaging dieline can look like a dense map filled with solid lines, dashed lines, panels, flaps, and all kinds of technical little marks. The simplest way to make sense of the dieline is to stop viewing it as a flat drawing and instead ask yourself what each line will do when the package is made. Some lines will define where the material will be cut away from the rest of the sheet. Other lines represent where the material can be bent without having to be cut. If the purpose of each line becomes clear, the structure becomes a lot easier to follow.
A cut line shows where the paperboard, corrugated board, label stock, or other material will be cut. A folding carton will have cut lines around its outside edge for the final shape; inside this edge, cuts might form a closure tab, a slot, or cutouts of a certain shape. A crease line serves a different purpose. It indicates where the material should be pressed or scored to allow that panel to fold without being cut. If you were to cut a crease line, you would separate panels that should stay connected; if you were to fail to crease a fold, it would resist, be misaligned or crack.
Start with the whole piece by identifying one panel that you are starting with, then the panels it is connected to. Look for the side panels, the top and bottom flaps, the dust flaps, the glue flap, and the closure tab. Try folding the dieline structure by running your finger over it. Any crease should feel like it would bend, and any cut edge should either be free or join to another edge after the fold. This helps you work out which marks form the structure and which are simply guide marks for the manufacturing process.
It’s worth cutting out your dieline and trying the mockup with just a simple paper carton. Use a key of symbols to identify the cut lines from the crease lines and label the main panels, glue flap, and closure parts. The cut line would just be around the outline and any internal cuts that are clearly specified. A crease line can simply be indicated with a ruler and a blunt point to indent the paper without piercing it through. You should slowly fold the mockup and see how the flat piece becomes a 3-dimensional package. Paperboard isn’t as bendy as plain paper, but it will work for a basic check on the order of panels and interpretation of the lines.
The last thing to watch for is how the lines meet each other. For example, a crease can end at a cut edge, it should not go on through every other nearby panel. Sometimes there are both cut lines and creases on a smaller flap, but in a small area. If it is impossible to tell if the cut or crease lines should follow each other, read the dieline legend or production notes to see how those lines should look. Also, be careful with assuming one style of cut or crease marks should be the same; every file can look different with different colors, dash styles or layers in the dieline. There is no definitive look to how these styles should appear.
While going through the structure of the file, keep the artwork separate from the packaging dieline to identify. Bleed is normally outside the cut lines so it covers the edge after it is die cut from the sheet. The safe area is where all the artwork and text should go so that it does not end up where the cuts, folds, seams and glue flap are. These are important guidelines, but they do not influence the dieline physically in the way that creases and cuts do. If you have layers in your layout file, try to turn structural layers on and off as you go to make the structure clearer.
An experienced dieline reader would expect that they have read the dieline well enough for the package to be foldable before cutting the actual paperboard and corrugated board. If they can identify all the panels, explain each cut line and show which panel will bend on a crease, they can indicate where the glue flap and closure tab should be to close the package. The best way to check you know what you are talking about is to make a mockup before going straight to the production sample and check your answers. It is easy to know you are getting better, but a better way is not knowing how long it takes to read a dieline, but how long it takes to be surprised when you start folding a flat dieline structure.