When it comes to T buckets there are alot of articles and books that cover them specifically. T-Buckets were the rage in the early 70's and because of this most Automotive interiors how to books cover them in some level of detail.
Cushions are typically made out of a 3/4 inch board and 3 inches of foam. Sides are made out of a lower panel with a foam block about 1.5" to 2" thick that runs from passenger dash all the way around to driver dash and acts as the upper seat back. And then a back filler piece is made that acts as a lower seat back.
This is a good project for a first timer but it is not necessarily easy. I would suggest you try this site and join in the forum. I have a T-Bucket but dont have any step images to walk you through it.
I am not affiliated with this site but have talked to them quite a bit and These guys live for t-buckets and can probably help alot.