This is my fave!
Tell us about the seats oh master upholsterer.
It's kind of involved, but if you take it step by step it's not too hard.
Ok... let's assume I'm doing the buggy's seat back.
1. Cut a piece of 1/8 thick balsa to the size of the seatback (I'll explain why balsa later).
2. Glue a piece of medium thick cardboard to the balsa and trim the edges to match the balsa piece. I use the cardboard that they use as a backing piece on yellow legal pads.
3. Take a piece of foam (I found pieces of foam padding in the sewing section of Hobby Lobby. It comes in various sizes and thicknesses, I bought a piece about 12 inches square by 1 inch thick)... cut a small piece to the size of the seatback, and about 1/4 inch thick. Thickness depends on the scale you're working in... the larger the scale, the thicker you'll want the foam.
4. Cut a piece of whatever material you're going to use as the upholstery slightly larger than the balsa seat back. You'll want enough material to be able to make "flaps" that will be glued to the back side of the balsa piece.
5. Spread some contact cement around the edges of the upholstery piece (on the back side, of course!). Spread the contact cement so that it covers the area of the upholstery material that will be folded over and attached to the back side of the balsa piece. It's not necessary to put any cement on the center area of the upholstery material.
6. Spread some contact cement on the balsa side of the balsa/cardboard seat back piece.
7. Now the tricky part. Let the contact cement on the upholstery material and the balsa seat back become dry to the touch. Now lay the upholstery piece face-side down on your work surface... add the foam padding piece centered on the upholstery piece, and finally place the balsa seat back on top, glue side up. Now, working along one of the long sides of the seat back, fold the upholstery "flap" up and over the balsa piece, gluing the flap down smoothly onto the balsa surface.
8. Now more tricky parts: With one long side of the upholstery glued down, gently press down on the balsa piece, compressing the foam padding... and while maintaining the pressure, carefully fold up the other long side "flap" and glue it smoothly down along the balsa piece. You get one chance with contact cement... once it makes contact, it's done, so you can't reposition the upholstery piece. You have to get it right the first time! I glue the cardboard to the balsa (step 2 above) to strengthen the balsa... without the cardboard, the balsa piece might crack when you press it down to compress the padding.
9. What you have now is the balsa seat back with the upholstery piece glued to it, and the padding in between... with the short sides of the upholstery piece still unglued to the balsa seat back. Now you cut slits into the upholstery piece at the corners (I cut 3-4 radially spaced slits per corner), then pull each small "flap" back around to the back side of the balsa seat back and stick each flap down one after the other, trying to create a "neat" corner. Once I get all the corner flaps stuck down, I usually flow come CA glue over the surface on the corners, to "lock" those corner flaps in place.
At this point you have a seatback that has been upholstered, with padding. If you did it right so far, the upholstery will be smooth and wrinkle-free in front. Now to the diamond tuft part...
9. Take a wide piece of masking tape (large enough to cover the entire seat back) and place it onto a glass surface (or on your cutting mat). Lay out the locations of each button. You have to decide how many rows of buttons the piece will have... take the height of the piece and divide by however many rows of buttons you want... in the case of the doctor's buggy I went with five horizontal rows, but the actual number will vary with the size of the piece you're upholstering. Lay those horizontal lines out on the tape, then on each line mark where each evenly-spaced button will be located. Remember, you have to "stagger" the rows of buttons to get the "diamond" effect. Now take the tape and place it on your upholstered seat back.
10. Now take a common sewing pin and push it into the upholstery and through the balsa/cardboard seat back, about halfway through... leaving about 1/4 inch of pin still sticking up off the surface. Repeat for each button location until you have a pin stuck into the upholstery at each button location.
11. Carefully pull off the tape, leaving the halfway inserted pins in place.
12. Now, using needlenose pliers, gently pull each pin from the back side, until the pin head compresses the upholstery a bit. You want the pins to create that diamond pattern. Make sure all the pins are pulled through the same distance. That's easy to check by just placing the seat back on a flat surface, pin points down, and seeing that all the points touch the surface. The friction of the cardboard and balsa agains the pin's shaft will keep the pins in place. I use balsa for the seat back instead of basswood, because the pins are easier to push through the balsa. Basswood is much harder and denser, the pins might bend rather than push through the wood.
13. Place a good sized plop of CA on each pin shaft where it sticks through the balsa seat back. Let the glue dry thoroughly, then snip the excess length off, close to the surface. Recheck the front of the piece to make sure all the pins are compressing the upholstery about equally. Push any pins that need to be embedded further and reglue from the back side.
That's it. You should now have one completed, upholstered and padded seat back.
Obviously the pins are bare steel. I painted the doctor's buggy seat parts with black acrylic craft paint after the pins were in place. It's much easier than to paint each individual pin head separately beforehand... although if you wanted the upholstery to be one color and the pins a contrasting color, you could paint the pin heads first, then do the upholstery.
Finally... the material I use to do upholstery is a leathery looking cloth-backed vinyl. It's very soft, very flexible, and has that embossed crinkly pattern that looks almost exactly like real leather. I found the material in the sewing section of Hobby Lobby. I bought a yard... that's enough to upholster dozens and dozens of model seats. It can be painted any color to create any color "leather" you like.