If you're using garment leather, its won't mould and stiffen all that well on its own because of the particular tanning process generally used for that type of leather . I'm not 100% sure if boiling will work in this instance but its certainly a very tricky process to get right and you'd certainly need a fairly substantial mould as a fair amount of pressure is needed to form the leather.
One option is to get hold of some veg tanned leather, you can often find small offcuts on ebay for a reasonable price, this can be wet formed reasonably easily.
eg
http://cgi.ebay.co.uk/VEG-TANNED-LEATHER-MIDDLE-SPLIT-1-3-1-4-mm-SIZE-A4-/160481986208?pt=UK_Crafts_Leathercraft_LE&hash=item255d78c6a0Personally I would probably make a plaster mould from a modelling clay positive (either water or oil based) and use that to form the leather.