Best way to trim the 55-062-2

Hi: im looking for Snap-in Glazing Bead, i believe the 6' KELLER SNAP-IN
Product Code: 55-062-6 . this is for my windows i have three of them broken. what do you use to cut them to size also. i'm a homeowner i like to do it myself, can you help. Thank you hope the pictures help.