All models of your army will be compared and sized vs. the ordinary creature. So it makes sense to start with that.
In this case it's the common Ork. It's 1:300 scale, so the ordinary Orc should be 6mm or slightly larger.
I looked at an GW Epic Ork with it's wide face and started doodling. Not on paper, but in the 3D program.
I used basic shapes for body and limbs. The result looked like an Fantasy Orc - Fantasy in the sense of Walt Disney not GW :( (Ork to the left)
So I thought about what general shapes should be visible on my Ork.
That would be muscles, muscles and muscles. To make them stand out I made them balloon-like. This looks weird in the rendering, but will, because of the scale, look ok when printed. I did also a test-shape for a new head. (Second Ork from the left)
The pose of the body didn't do much to convey the image of an Ork, so I tried the standard pose of the 40k Orks - you know the Gorilla pose with the hollow-back and the bum stuck out.
The Ork is plausible, because it has the basic physiognomy of an human and the standard pose reminds you of an sumo-fighter, gorilla or 40k Ork.
I would want to be able to pose the model. There are basically two easy ways (I'm aware off) to do so. One is rigging. You create a skin, put bones within and connect the skin to it. When you move the bones the skin moves with it - the important thing is the skin moves changing it's shape. The advantage is that you can get really good and natural results. Drawback, you have to invest some time to do so and if you don't the results can be not so convincing. Also the thickness of limbs can change, which isn't usually a problem, but in this scale it can be a question of to snap or not to snap or even print or not print.
I took the second approach, which is probably not possible with all 3d software packages. The body is segmented and the segments are put in a hierarchy. So fingers are below hand, hand is below forearm, forearm is below elbow, elbow is below upper arm, upper arm is below shoulder, shoulder below torso etc. When you turn one segment all segments below it will turn with it, keeping their relative position. Also when the segments turn or move, their shape will not change - the segments will keep their size under all circumstances.
When looking at the renderings keep in mind that the printed and painted models will look different. Features will be more blurred and less pronounced not only because of the printing and painting process, but also by the fact that the miniatures are only a tenth of the size of those renderings (might vary depending on your screen).
The body looked ok, but the Fantasy head didn't really fit. So I refined the quick head and that led to the final design dubbed Shaun.
Shaun went through different revisions, one of the reasons the teeth were too small when printed and the eyes did not hold enough paint/wash to stand out. I enlarged the eyesocked and the "round" teeth got replaced by "angular" teeth, which have more volume with a bounding box of the same size.
There are other variants, with a bikers-helmet, glasses, goggles and hair squigg.
You will realize that there is no neck and the head is rather deep and is "shoved" into the body. For one that looks orky, but it also ensure that the head is sitting firmly on - or better in - the Orks shoulder.
All Orks in the army are a combination of the standard body and one of those heads. There are some simple accessories to be added to some. Remember too fine a detail will not show/print anyway and sometimes the parts have to be made ticker and/or embedded into the body to prevent the from breaking.