Identifying something using a key, and inventing a key, are very different exercises. In the first instance you are trying to fit a single specimen into a category; in the second, you have to characterise a large number of different categories with sets of exclusive distinguishing features.

The first consideration is, what is the key designed to cover? It might include all skulls or be restricted to the skulls of British mammals. You might want to design a key to blue butterflies, or to the wormlike animals in estuarine sediments. Keeping in mind the scope of your key will help you to include only those decisions that are relevant to it.