I usually start with the side of the face (unless it's the back of the head) and shape off the top of the head with a circle so I can place the eyeline, then place the center line so I can work out the chin. It usually looks like a rounded cross attached to a circle or the face of a shield. If the face has an open jaw, I also place two guidelines for the top and bottom teeth so I can place the lips.
Your characters seem to have enough diversity to make them feel unique. If you're having trouble with drawing from the same angle, try flipping the image once you have your guidelines placed to get a feel for drawing them on the opposite side. Try to get a good system down for placing guidelines, which is usually a circle with an eyeline and center line. The better you know these shapes, the more angles you'll be able to draw them in and then you can use them to work out face shapes from different perspectives. When I first started drawing faces, I made a basic head-shape model out of clay to draw it at different angles. Once I had a better feel for the shapes, I could start playing around with face styles, expressions, shading, and age. For a while this might be a very tedious process of practicing with guidelines, but after a few practice sessions you'll be able to freehand things better and work out angles faster. I also found that working with a specific non-character (just a simple random design of a human face that you can use for practice) is better to work with than characters you usually draw because it makes you worry less about how perfect the finished product looks. I made some random human design just for practice sessions so I can play around with the consistency of my drawings and it's helped me build up enough experience to work with my own characters without getting frustrated. You can still start on whatever part of the face you like, but having a better understanding of face shapes will help you improve your range of faces :Y
