How to Draw a Sergal
Step 1: Draw a triangle
Step 2: Draw a parallelogram
Step 3: Add details
Step 4: You did it!
Sergals are really just cheeseheads.

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How to Draw a Sergal
Step 1: Draw a triangle
Step 2: Draw a parallelogram
Step 3: Add details
Step 4: You did it!
Sergals are really just cheeseheads.
This is really simple and doesn't really touch on the expressions bit that you're having trouble with, but since expressions require the face to work, it's a good place to start! :) Granted, this is something simple I whipped up in an hour or so, so it's not extensive or detailed. It's just little things that come to my mind first when I'm having trouble differentiating gender between characters' faces.
Some more detailed notes:
Male faces tend to be more angular and sharper. Things like a more rigid jawline and a sculpted chin.
Female faces, more rounded and softer. Their jawlines usually aren't as defined as a male's and their
Applying the angles vs. rounded rule to facial features is an easy way to define gender. Rough angled noses vs. soft button noses, for example.
HOWEVER! Don't be afraid to mix things up a little bit! Sticking to that rule can really kill diversity in the way you draw faces. Go ahead and curve up your manly faces or to throw angles onto your feminine faces. There are always other ways to tell what gender a character identifies as (body shape, anatomy, clothing, hair, stature, etc.)
Going on a slight tangent here, but don't be afraid to research some things and ask yourself some questions. What generally defines male and female faces in real life? How do your favorite artists differentiate genders between their characters? Things like that, you know?
Speaking of research! Here's some VERY helpful links to help you out on more facial business and especially expressions!
How to draw all sorts of crap (including faces!)
Drawing emotions with Scott McCloud
Simple expressions with alexds1
Lackadaisy expressions (probably the BEST tutorial on expressions)
Hope this helps!