My everyday work doesn't involve being hands-on with Azure, so AZ-300 was not a straight forward path for me. To help me with presales activities, it was very important for me to understand the product offerings in detail. 

The prep took me around 45 days with the learning, hands-on labs and practice tests.

I started with cross-referencing the topics covered under skills measured with Azure documentation. These documentations are quality materials with updated instructions for the hands-on scenarios. You will get a very good knowledge of architectural principles and considerations. This article ravikirans.com helped easily links to relevant materials. 

I did subscribe to Scott Duffy course on Udemy. You will learn how to perform various tasks, however not in-depth for the objective I wanted. Some sections, specifically demo may be outdated.

Create an Azure account and try out the tasks yourself for a better understanding of the products. Specifically around VNet setup, peering, load balancers, gateway, backup & recovery. 

Finally, I practised using Whizlabs. A very good source with explanations to answers.