Time for some more easy work.
The seatbacks are held in position by some P3 hinges. Vans wants you to install hinges at three different positions. As different pilots have different leg lengths, it might be necessary to adjust the seatback position.
It's a bit difficult to evaluate if these positions are going to work for me as you don't know yet how big those back and bottom cushions will be so it's a bit of a guess if this will work or not.
I do trust Vans that in their plans, they accomodate for a lot of the pilots and as so many kit are finished and flying, I assume I'm no different than the average pilot.

One word of advise before you start though !
When you cut your hinges, separate them first before you cut (so pull the pin out and measure one part at a time. (Ask me how I know @#@#&&!!!)
when you measure the distance for the seat plate as shown in the image below and cut both parts together, you will end up with two eyelits removed on the other side as the side you're measuring now has eyelits on each side.For the top hinge attached to the seat back brace and the bottom seat back hinge that's a problem. Now you're stuck with a lot of hinge that is just a half an inch too short. I guess you see where this is going...
Vans provides just enough stock material in the kit. So what to do next ???
Yes, order some more hinges at Vans and pay the price for your stupidity in transport cost and import taxes...

Anyway... you only find that out later as you build the seat back so be carefull here.

First step, measure the distances as shown on the plan and copy them on the aft seat floor. Draw the lines. Also draw center lines on your hinges.

Measure the location of the holes on the hinges as indicated in the plans. Pay attention ! They holes are not evenly spaced so measure carefully.
Then you copy the holes on the hinges by clamping them back to back and using the previously drilled holes as a template.

Remove two eyes from the center of each hinge. This is where the pins will be inserted to attach the seatback. Pin goes in from the center to the outsides.

Now, clamp the hinges in position and drill hinge to the seat floor. Most important is too measure carefully once more the distance from the center of the holes on the outboard edge to the start of the hinge.
So NOT from the edge, but 7/8" typically from the center of the holes to the edge of the hinge. Many buillders make this mistake. Also, some builders are facing issues later with xthe seatback position where the seatback interfears with F-757 plate when they are installing a tipup canopy. If you are building a tip up, better search for some more advice on some other builder sites.
I'm building a sliding canopy which has an edge removed from this plate, so I will be fine with the plans measurements.
Also check distance once more from front edge to the hinge so that your seat back will sit perfectly horizontal.

Do this way of drilling only for the first 2 hinges. (forward)

The last one at the back needs some more attention.

The last hinge has 3 extra holes that are common with the predrilled holes by Vans. Make sure you check alignment or you will end up with edge distance issues if you didn't check.

How this works is actually easy, you first clamp the back hinge on the line that you have drawn before and in line with the other 2.

Now turn over the seat floor and check through those 2 commong back holes if your centerline on your hinge is visible through these predrilled holes in the seat floor.
Reclamp if needed.

With all fixed in position, it's now safe to back drill those 2 holes using the holes in the seat floor as pilots.

Now that those 2 are locked in with cleco's, turn around the seat floor and drill the rest of the holes using the holes in the hinge as pilots.
And that's it.

Did the same procedure on the passenger seat floor.
Deburred all the holes on seat floor and hinges and that's it for today. All ready for priming.