Use an orthographic (not perspective) camera, positioned 30 degrees above the horizontal, and 45 degrees (halfway) between the two horizontal axes, aimed at the origin (0,0). Or, if you want true isometric rather than pixel-isometric, position the camera 35.264 degrees above the horizontal. The camera can be as zoomed out or zoomed in as you want, it can be at any distance, as long as the angles are maintained, because orthographic cameras do not have perspective distortion.
Wikipedia has an illustration of this set-up (for true isometric).
Keep the light source the same on all the pieces, and make it a directional light, not a point light. Directional lights can be placed anywhere in the scene, and their angles correspond to the angle of the sun in the sky, essentially. The horizontal angle should be 90 degrees leftward of the camera (so, 45 degrees in the opposite direction of an axis from the camera) The lower the pitch of the directional light, the longer the shadows will be, so make sure you keep it high enough that the shadows aren't too big. The models should feel "self-contained", and that means relatively small shadows.
Since I am not confident in my text description, here's a
sketch of the camera and light set-up that might help.
Beyond that, it's just a matter of making a model that looks good. When working on your model, always keep a view open to see what it looks like in the "isometric" camera, since not everything will look good at that angle.
It's possible that there is no actual lighting in the examples and the shadows and highlights are painted on the textures, at least in Retronator's examples, where the shadows do not really match the the details of the models. They might've gotten some initial shadows by rendering an untextured, simplified model with basic lighting, and using that as a guide to paint the shadows on the texture, and adding more details to the model later. Leaving out shadows from certain details might seem like a mistake, but it actually serves to keep the visual noise down. Since these are seen small, small details don't read well and just look like noise. Simplifying the shadows so that only the main volumes cast shadows helps the scenes look cleaner. Even if the shadows are entirely hand-drawn, the artist still understood the directional light source and its direction.
