Get some old lasts (ebay is a brilliant place to look - but do search for other terms, foot molds, foot forms shoemakers molds etc. as well as lasts because most people don't know the correct terminology).
Set your own still life up and practise sketching them from different angles. Feet are a weird shape to get right - if you practise drawing the lasts,you will soon notice an improvement in your footwear renderings.
We had to do this in the first year of our footwear degree.
When I am in a hurry I have been known to trace over a jpg of an existing shoe in Adobe illustrator (I sometimes do this in the factory,with the last I'm going to use) Because I am so often pushed for time, it saves me the time of spending ages getting the proportions right before I sketch the design out.
Remember as a designer it is your ideas that count - use all that is available to you and all the tricks in the book to get your ideas on the page in an accurate manner. Cheating is completely and totally allowed.