If you are wanting to actually do your drawings by hand, the first thing I would do is invest in a graphics tablet, if you haven't already.
The Guild actually has a really good collection of Photoshop tutorials that I can personally recommend.
As for resources... Everyone is different when it comes to maps, but for me one of the most important tools in my trade are good textures. My own maps lean heavily into the antique parchment type of aesthetic, so for me my large collection of high definition paper and parchment textures are an absolute must. A lot of my best parchment textures come from this amazing
collection by CoyoteMax, I've also made some
myself, and the rest I've bought from Shutterstock. Textures.com also has a really good selection of seamless textures and patterns for things like rock, dirt, vegetation, tiles, water and the like for more realisctic, satellite style maps.