buy yourself a decent cable cutter.
buy some cable housing and ferrules to cap the ends of the cut housing.
Hold the housing against the frame to find the right length and hold the spot you want to cut or mark it. cut it, then cap the ends with the ferrules and place the ends into the brake lever/frame stops/caliper, thread the brake cable through the housing, set the cable at the correct tension, tighten the bolt that holds the cable on the caliper and you're set.
you might have to open the ends of the cable housing where you cut it. sometimes the inner plastic sleve gets crimped. some cable cutters have a prong you can stick in the end to open it up, or just use a finishing nail. that will make threading the cable much easier.