A packaging dieline can seem complicated because different kinds of marks are all drawn on the same surface. Some are for cutting, some are for folding, and others are for supporting printing and joining processes. The best approach to understanding a dieline is not to think of it as one complicated design. It is to think of each mark as addressing a single production question: where is it going to be cut, where is it going to be folded, how much extra is going to be printed, and where is it going to be glued.
The cut line marks the outer shape of the package as well as any internal cutouts. When the die is ready, everything outside of the cut line is removed. The cut line can define the outer panels, the tuck flaps, the locking tabs, and the glue flap on a simple folding carton. Make sure you do not confuse every line you see with a cut. You would cut panels that are supposed to be folded together if you were to die cut along a crease line. If you were to overlook a die cut, then the tab or closure would not operate correctly. To ensure accuracy, consult the line legend or production notes in a PDF proof rather than depending on colors alone, as dieline colors may differ between each file.
A crease line tells the paperboard where it should be folded. A crease line may have dashed lines, dotted lines, or different line colors depending on the template. When you crease paperboard, you are compressing the substrate so that it will fold in a predictable path. Grain direction affects results because you may end up with a messy finish or surface cracking on some substrates if a crease is made across a grain direction that is inappropriate for the stock thickness or paperboard coating. To test a crease line physically, print out a paperboard carton template, and lay it on a cutting mat. Then you can use a blunt scoring tool to create an indentation on the paperboard before you fold it.
Bleed is non-structural. It is an extension of the printed artwork beyond the cut line. Extra print extends beyond the cut to avoid thin unprinted strips where the cut may shift within tolerance. Any background color or image that should cover the edges of a panel must extend into the bleed. Text, logos, barcodes, and other critical content should go in the opposite direction, into the safe zone away from the cut line. Do not extend a logo out into the bleed, which does not increase the likelihood of it surviving; it increases the chance that it will get chopped off.
The glue area serves a yet different function. A glue flap is a strip that joins two parts together in a carton design. Adhesive may be applied to a glue area only. This sometimes means the removal of the background artwork or printing a different version of the artwork, but not always, as the specifics depend on the type of glue, the type of ink, the type of varnish, whether a laminate is applied, and the gluing technique used. You could also be printing over a panel that overlaps a glue area. That may result in part of your text going into the inside of the closed carton. You need to identify the panel that goes on top after folding and then you can find out the panel that will receive glue.
You can make one basic sleeve or carton dieline and draw each feature you will want to see in black ink with a sharp pencil. Start by outlining the cut line in black. Next, mark out the fold lines in black. Then fill in the background color for the bleed, and then lightly label out the safe zone and the glue flap. After that, you are going to print out your artwork on paper and assemble it by hand, so it is helpful to hold an open one by side so you can compare the die to your assembled carton. It is helpful to hold an open version of it alongside it to compare with your assembled carton. This shows you how each of your panels sits inside your package. You can identify the position of hidden seam, the direction of your folds, and the positioning of your opening.
Always check a dieline for print readiness in the flat and in the folded condition. Verify whether the artwork extends beyond the edge of the fold, that all text and essential graphic elements are well within the safe zone, and that no critical text runs along a fold, and ensure the glue area is suitable for the required gluing method. It is not necessary to learn every specific line style used by every printshop. It is more critical to learn exactly what each symbol means, and then verify all of your assumptions before your artwork enters prepress and converting.