You strip back around 6" of the insulation, wrap it around the metal part to be magnetised, then WEARING gloves touch the ends of the wire one on negative one on positive, keep dabbing on the wire will HEAT up alarmingly ie glowing, so much caution.
A simpler way is to buy a magnetizer/de magnetizer for around £3 on ebay etc, intended for screwdrivers etc, but works on anything metal stroked with it.
Another way to do it is to use a fastener of the same thread that is about 1" longer and use that as the sump plug after magnetizing.
You will though this way need to lathe/machine down the shaft of the fastener, which you may not have access to.
The magnetized plugs you can buy are good, but as usual some are a piss take price imo.
tabetha