Think with a pencil in hand

A student of mine the other day was in a classic rut - one I see in nearly every student I have. They were overwhelmed by ideas, and found themselves unable to translate them into a tangible, visual result. They had all these great theories about what their art should be communicating, but they were stuck on how to get across their ideas. 
This resulted in them staring at their notebook, head full of a million thoughts, without a single sketch to speak for. I think every artist has struggled with this at one point or another.
Most artists prefer to sit and imagine their perfect work over making it. Heck - I do! It’s so much more comfortable than getting down to the gritty work of trying to make the vision a reality. But why? Why is this habit so pervasive in the art world, and so difficult to kick? Why do we tend to opt for day-dreaming over doing? And is it really so bad?
I believe we do this because we subconsciously know that when pen is put to paper, that idea becomes real, fallible, and flawed in comparison to the perfection in our minds. Reality can never hope to be as immaculate as the images in our heads. 
It feels safer to let those perfect ideas stay up there, never existing, absorbing our precious time in daydreams that never lead to anything tangible. The worst part about this trap is that it feels productive. You really think you’re working through problems, making progress on your vision…. But in reality you’re not. Art is not composed of daydreams, art is composed of action, and without action there will never be any art. 
So is this habit really so bad? Yes, and I would go so far as to call it tragic. It is the cancer that causes millions of creative dreams to remain unfulfilled; it’s the reason someone wants to write a book in their 20s, and by the time they reach 60 have nothing to speak for but a general sense of longing. It’s what makes established artists stagnate, unknown artists devoid of meaningful work, and I would even say it’s a cause of depression and anxiety. Or at least, it was for me. 
It’s really that bad.
So what’s the solution? I’ve thought about this a lot, and the only effective thing I’ve found is as simple as it is blunt: Just. Fucking. Start.
There is nothing else, there is no other option. You just have to make the decision to do it, and then you do. 
Force yourself to sit down and make sketches of your idea; draw blobs for the figures, scribble in areas of the background, make a stick drawing of the sculpture, etc.
When you notice that your mind is buzzing with murky concepts, the act of making terrible sketches solidifies it in the world, giving you a touchstone from which to start, to improve, to rehash, to build. 
Once you have that down, look at the sketch and think about the next obstacles to making the piece. Write them down (don’t just think!), then, beneath those, write out potential solutions. Make a timeframe for implementing those solutions, deal with them within the next week. Action, action, action.
In the act of drawing, you actually bring this idea of yours into existence. The important part is that you’re thinking with your pencil. You’re thinking in the same way that’s necessary to actually make your idea. 
Consider this: if you were to die today, all those thoughts in your head would die along with you. They will be gone, never to be shared with the world, never to exist. Only when you turn it into something tangible does it live on. 
Make bad sketches. Start on that grand idea you have had for forever. Start right now.