Switching from drawing what you see to constructive drawing.

[lurch]

Hi,

First post here, so please bear with me!

I've learned to draw from observation to a very mediocre level, having been taught to draw what's in front of me by measuring and breaking it down into shapes etc, and it's always bothered me that I can't draw from my imagination.

I'm trying to address that, work on my perspective and anatomy, draw more constructively, but I'm having trouble switching to this method.

When I'm drawing say a head from reference, or from life, should I be trying to break it down using spheres etc to build it up, more like a loomis head?

At the moment I would just starting drawing, check things to see how they scale/line up against other things, and keep going, tweaking until it's done.

I can't get my head around how I would build an accurate drawing constructively.

Same for perspective, I've tried placing the vanishing point when drawing on the train, and generally ended up with a less accurate drawing due to mis-placing it, would I draw 'through' shapes when drawing constructively, ie, draw a box, rather than just the lines I can see of it?

Sorry for the big post, I'm keen to start learning how to construct my own stuff, I'm just struggling with this concept.

Thanks.