Here is an interesting statement by Calder about his process in making mobiles. I find it encouraging to know he didn't always design them in great detail beforehand!

On The Making of Mobiles

Alexander Calder

I used to begin with fairly complete drawings, but now I start by cutting out a lot of shapes…some I keep because they’re pleasing or dynamic. Some are bits I just happen to find. Then I arrange them, like paper colle, on a table, and “paint” the – that is, arrange them with wires between the pieces if it’s to be a mobile, for the overall pattern. Finally I cut some more of them with my shears, calculating for balance this time.

I begin at the small ends, then balance in progression until I think I’ve found the point of support. This is crucial, as there is only one such point and it must be right if the object is to hang freely. I usually test out this point with strings to make sure before bending the wires. The size and angle of the shapes and how to use them is a matter of taste and what you have in mind.

To most people who look at a mobile, it’s no more that a series of flat objects that move. To a few thought, it may be poetry.