First, make sure that your iron is on the highest setting possible. When it’s at its highest temperature, you can then hold a paracetamol tablet with tweezers and rub it over the soleplate.
You should be able to see the burnt, black marks come away as you wipe, but you may need to repeat this process with another tablet if you have particularly stubborn stains.
Finish by wiping the soleplate clean with a microfibre cloth.