Here is how I draw horses:
First, I place the red lines, which is basicly the skeleton. Just a few circles, sometimes I already connect the 2 large circles of the main body.
Then I add the blue lines, which is the outline of the body. As you see, the lines aren't perfect. So i should be adding a new layer with the final lines.
My main advice is to start by placing the skeleton, than add the rest. Don't add too many details with the sketch, those will come when you draw the final lineart.
When adding shading, decide where the light comes from and work from there
