As this is intended to save you money, buy only the materials you are sure you will need. If you run out then buy more. It is always more cost-effective to gather your own materials rather than buying them. Avoid having extras, but if you do, try to sell them back on the Auction House.
Alternate steps are marked in green. Steps in which there are more than one option are marked in blue. If you consider these options, obtain all the standard materials but disregard the total numbers in favor of the option totals.
If three Ruined Leather Scraps are cheaper than one Light Leather, it's actually a good idea to convert as many scraps as possible to light leather, which brings the skill up to 34, and start creating Handstitched Leather Cloaks only then.