It takes a bit of time and patience, and it's only recommended of you really want to. You start with cleaning and sanding down rough spots; then carefully fill in with a vinyl/leather compound, let it dry. Any tears need to be glued to a patch from the under side. It has to be done neat, smooth and even with the surface. Then you dye it (or it will be left as a sort of white rubbery filler). Some products require you use heat, like a heat gun on a low setting or hair dryer. The dye often comes in primary colors and you need to blend them to match the color like on painters palette. I guess black should come as a ready made option.
All in all, I would rather reupholster the seat if you can't find an acceptable new replacement. Go to a store that sells pvc - faux leather material for cars and motor bikes. They usually have smooth finishes and some leather imitations, but I have yet to find the tough materials with sort of puncture hole pattern that was common in the 80s and early 90s. It's not expensive, sold by the meter (or half a meter, any length cut from a bolt). New upholstery material stands up to wear much better than any repair. Accept signs of age and wear, and replace in good time before it all falls appart. Seats can last a long time, but before you spend time and money on it, make sure all parts from seat post to padding is in good condition.
Last edited by Mickey2; 07-23-19 at 05:22 PM.