First thing I suggest you do is bolt in the main bar, then refit the seats, and decide if you really want this rollbar. Okuyama bars can rob quite a lot seat travel. It it doesn't suit, no real harm done, except some extra holes in the floorpan.
It doesn't stop there; for some reason, they made the rear stays quite obstructive to the seats; inparticular the bolt joining the parts up. I managed to trash a brand new Shoei helmet using a steel 6-point Okuyama at Curborough once.
On the Okyuama bars, the main bar footing should fit right into the corners; you need to cut the body sealant here right back to metal, as its unevenly applied at the factory. You might also find you won't be able to access the tenax stud by the door; you you can do, and it works, is to drill and tap the bar for a tenax stud, if you like using the hood cover. On the rear shelf; obviously remove the unpainted section. With this bar, only a little bit needs nibbling out to clear the bar. I used aircraft snips (£4 B&Q, for a full set that will work just fine for this job; right and left cutting snips are essential). Like you, I had no clear install instructions, so it was trial and error how much to take out. When fitting the side bars, check if the bolt interferes on the stock seat; I had to remove the small plastic finishing part off the seat to allow it to recline. I would pull the carpet back, and offer up the bar, and drill. Check fit; this will give you a good indication if you've screwed up on the main bar; the plate should be flat to the floor.. Now remove the bar, and refit the carpet. Now you've got to judge where to make a slit in the carpet to pass the bar through. Fit the bar through the carpet and bolt up. This makes for a neater solution than cutting the carpet around the bar once its in; if you do this, the carpet is liable to pull out of the trim pieces, and look tatty (even more likely on a 96 model with the thin, cheap carpet; hot tip; if you need a bit more carpet in the driver's footwell, loosen the foot rest, tug the carpet, and retighten the footrest.). You won't be able to bolt the bar through the carpet; the thickness of the carpet means it won't sit flat.
Drilling the rear stays is a little tricky' you really need a long HSS bit for this. Make sure the stays are pointing sufficiently outboard to clear the frame box section. In board by just a few mil, and you end up drilling into the box section itself. Its that tight. On the drivers side, you have to make sure the main loom isn't fouled up. I would go to the effort of painting the drilled holes; don't use Hammerite. It won't take to the metal well. Also use some body caulking between the bar and the body, to cut down NVH. Don't use it exterior to the car, as it absorbs moisture. Chuck away all the fixing bolts that came with the bar, and get new ones, with lock nuts. Bolts stretch, and I wouldn't reuse old bolts for this. You can get rubber bolt head covers to protect your head for the rear stay bolts.
Finally, dowse the floor plates and nuts in a good underseal wax, like dinitrol.
I followed all that, and 5 years later, removed the bar, to find the holes perfectly fresh.
Take all the seats out when you do this, as well as the centre console to save marking it, as you'll be using the cabin as a climbing frame during the install.