Several things. First, no potato should ever go anywhere near a microwave in any guise. Second, no oil of any description should ever be applied to a potato about to be baked. Give the spud a good scrub and excise any bad bits. When almost dry, smother the skin with salt (no pepper - ugh), letting the excess drop off. Preheat the oven to about 200C (400F). Prick each spud several times with something pointy. If you don't, the spud may explode in your oven. Place the spuds on the oven shelf. Don't put them on a tray. SRS's notion of skewering the spuds to get them cooking faster is something I've tried, but I've never observed any advantage.

Finally, you don't get a decent jacket spud if you start with a bad spud. If you start with an organic potato you'll almost certainly get a good result. Most inorganic spuds are useless owing to their high water/low dry matter. Your baked spud will be mushy and soft, not at all whst you want to achieve. My favourite variety is Nicola, though Charlotte also gives good results.