I recovered a vinyl seat with aprox 3/16 thick leather.
Used contact cement(plastic laminate type) to adhere over the old vinyl.I dry cut the leather with 1/2 in.overlap to roll under the seat.You really must pull tight with pliers to achieve a smooth surface.There is usually a block under the back of the seat to use a few brass screws,tacks.The front nose of the seat you will have to slit to overlap,splice and a few 1/8 screws or tacks.
A lot of Black Powder Gun shops seem to have an abundance of leather.Next we will discuss on covering your drops with the same color leather so buy extra,its not very expensive.