[SamUK]

Marine vinyl, staple front and rear, heat gently with a heat gun (or the wife's hair dryer) then work your way Front to back. You'll need to work both sides at the same time to get a great finish, a few staples down the left then a few some the right and so on...

As @mrbb suggested, stainless staples are a good idea if you want a good job. If you plan on using a manual (hand) stapler it may be worth gently warming the staples before firing them in. Speaking from experience, never ever do they all fire in all the way, you end up tapping a few in with a small hammer.

 

[SamUK]

I have a concern about using a piece of marine vinyl. My pre-cut seat cover featured a sewed-on portion for the rear of the seat that accommodates the sharp angle at the rear of the seat. I think if you just use a flat piece of vinyl, you are going to get wrinkles at the rear of the seat.

Also, as SamUK said, the staples from my manual stapler did not go in all the way. I used a punch and hammer to finish them. Not a big deal.

Take your time and you get no wrinkles at all, marine vinyl stretches more one way than the other, getting the stretch directional on your seat is key to getting the job done perfectly.
I think replacement 'off the shelf' covers are made with 'ease of fitment' in mind, they are a lot easier to fit than using marine vinyl but the downside is the material is usually thin so doesn't last as long.

 

[SamUK]

I use shoe goo to fix small holes. Won't hold up for more than about 6 months, but then you just slap some more shoe goo on there.

I just ordered a $25 cover on ebay for a 420 seat I have that needs recovering. So staple front and rear in place first, THEN stretch the sides out and staple them down?

That's a good price for a replacement cover actually Jeep. Yes, exactly that, just work your way front to back. Put a FEW staples in as you go, once your done take a look then fill in the gaps of your happy with the outcome. Try not to over stretch the vinyl too, it's very easy to get 3/4 of the way through the job and realise you pulled a bit at the front to tight!