I have indeed fitted a seat cover in the past.
Would I do it again....not a chance!
Replacement seat cover kits used to be common, and fitting them was fairly easy. Fitting them so they were tight, even and looked good was absolutely not. I was fitting a cover so I could use the seat...the old cover having been ripped...gaffa taped...ripped more...taped more...and so on until it was an eyesore. so pretty much any result was going to be better.
Only time I got a result I was proud of was on a plastic seat base using a staple gun. Minimum 4 arms required.....
I have the utmost respect for professional trimmers, whose skills I lack!