I agree with what's been written in the John Healy article and would add a couple of thoughts based on replacing seat covers over the years:-
- instead of using clothes pegs, I would invest in a set of 12 or more spring clips which have a greater clamping force and allow the cover to be pulled taught as required, see This Link
- I use aerosol carpet fixing contact adhesive which makes application of glue easier.
- make sure your seat base is in good condition and doesn't have any rust or cracks, fix these first.
- if the seat foam has perished, it's perfectly possible to buy new foam from upholstery suppliers and trim it to shape. Use contact adhesive to glue it into place on the base.
- to help the new cover fit the foam you can add a suitably sized plastic bag over the foam, trimming off any excess, this helps reduce friction between the foam and cover and assists when pulling the cover tight, make sure the plastic bag just covers the foam and doesnt prevent the cover being glued to the base.
Hope this helps