I have layered corel template files that I use for making banners.
Layers:
Grommets (for customer proof, does not print)
Grommet Dots (printed small dots that show where grommies go. No measuring!)
Stitching (for customer proof, does not print)
Hemline (where we fold to do the hem. Also is thhe finished dimension)
HemBleed (1" outside contour of Hemline... contains b/g color or image full bleed)
HemMask (a mask that obscures the area between the HemBleed & the Hemline. Used to take a screenshot to proof the banner so WYSISYG) Does not print
DimensionText (displays dimensional information for customer proof. Does not print)
These all go in Master Layers, then content goes in regular layers. Over the years I have accumulated practically every size so now it is easy. Just "open new file from template" then select the size banner I need, and plugin the content. Grommets are already laid out, stitching is there.. When it is proofing time I turn on the grommets, stitching, and hem mask layers and take a screen shot. When it is printing time I turn them off and turn on the grommet dots (they are smaller (.25), and color depends on b/g layer for contrast)
So, to answer your question... I would just use a straight edge ruler and a box knife to cut along one of my printed lines.
Not a printed banner? Then you have to use a ruler to mark your measurements, then draw a line with a blue stabilo pencil (or a white one if a colored banner)... then cut as described.
Example.. for a 20" banner coming off a 24" roll.. You mark 4" in from one edge. Then slide down to the other end and mark another 4" spot. Then use your straight edge ruler (we have a 28" one and a 52" one) to draw a line connecting those two points. You can also use a carpenters square (sheet rock square) to make the 90° lines. The stabilo lines just wipe off.