It's REALLY simple . . .
Arm yourself with a good quality utility knife . . . NOT a "box cutter"; but a utility knife like the ones used to score and cut drywall . . .
Put in a NEW, SHARP blade . . .
Get yourself a "straight-edge" 2-3 feet long . . .
Mark your lattice where you want to cut it . . .
Now, place the straight-edge on that line . . . and repeatedly "score" the lattice about 8-10 times. I find it easiest to score ONE segment (open edge to open edge) at a time . . .
Once the lattice is adequately "scored", bend the lattice "backwards" at the "score" . . . just the way a drywaller would do . . . and the lattice will SNAP apart . . .
The cuts are sooooooooooooooooooooooooo much straighter and neater than those made with a jig saw . . . or a hand-held circular saw . . . and IMHO, A LOT easier . . .
BTW, I've been cutting vinyl lattice this way for over 15 years . . .
Hugs & wags,
Al & Pelli in OH