We start by releasing tension safely, usually by unwinding the spring with winding bars rather than cutting anything under load. Once the door is de-tensioned, we pull the old cable off the drum, check the drum groove and bottom bracket for wear or rust pitting, and inspect the other cable even if it hasn't broken yet, since cables on the same door usually age at the same rate.
We replace with a cable matched to the drum size and the door's weight, not a generic length cut to fit. If the drum itself is worn or the wrong pitch for a clean cable seat, we'll flag that rather than force a mismatch that fails again in a year.
Before we leave, we re-tension the spring, run the door through a full cycle by hand and by opener, and check that both sides are tracking even. If the bottom brackets or rollers show heat-cracked plastic or rust from the same age and exposure as the cable, we'll point it out, but we're not going to sell you a part that isn't failing yet.