I would think that all makes of roller blind were virtually the same in construction. If you do have to trim them to the required width, measure the amount to be trimmed off on the back of the blind - faintly, in pencil - at regular intervals down the side(s). Then with a long ruler, join these markings (again, faintly, in pencil). This will give you a nice straight line to cut to with a sharp pair of large scissors. If you can see the pencil after the trim, remove with either a rubber (eraser) or a chunk of white bread.
If the blind has a distinctive pattern, be wary of only trimming one side - you might end up with an unbalanced pattern.
If the blind is not fabric but say wooden or plastic slats, you may need to trim each end with a sharp stanley knife. Once again, faintly mark your cutting line on the back and remove the pencil markings afterwards. You might also want to lightly sand the edges down after cutting to smooth the surface down.