I know this is very individual, but personally i had probably sketched it up in a 3D cad program first (But You can of course just go directly on a cardboard mock up model and cutting it out by hand for example and only change the parts that need to be adjusted, personally i use Lego and Meccano a lot for testing out ideas).
It requires a bit of mathematics, but it is technically not that hard to design cogwheels.
Cogwheels from scrap almost newer fit to each other, so here You will have a massive advantage if You can design them. 
I would have a better time if i had a small 3D printer, however i did get a book called "Steampunk: Gear,Gadgets and Gizmos, a makers guide to creating modern artifacts" by Tomas Willeford which goes into depth about some projects that i could get into.
one of the ones was a phone holder that is from a model T Ford ignition Coil that is a wooden box essentially.
i have ideas to make it less an enclosure and more of a phone case i can have, a clip on one that allows me to remove it when i need to.
essentually i plan to take the T model (i found plenty on e-bay) and reduce the boxes size to be a more thin wooden brick.
the phone will already have a phone case on it but it will be one with a clip or hoop that i can attach to some string inside (with a place in the back of the box to remove the string and tie it) this will allow the phone to stay inside with the ability to remove it when i want, afterwords the box front face will have hinges and a latch so i can use it to access my phone without removing it from the box.
Thomas's 2 books are a good place to start. Get the kids book as well. The reason is that there's good projects in there and the only difference is making it with leather and better crafting.
Key to that is this, what kind of crafting experience do you have? I am thinking next to none, primarily because an experienced crafter tends to already be diving in to the next project. You're hesitancy signals you're not there yet. That's OK. You need some successes, and Thomas's book will help.
Now let's examine that T-Case project (I have the same book). He's using a grimy original object, disassembling and cleaning it, before adapting it. You can skip that part IF you are comfortable making a box from scratch. It's not hard if you have tools and a bit of woodshop experience. And even with Thomas's version, you still might need to cut some wood.
The basic idea is that you make a box frame that fits your phone (and case), perhaps with some shims. It'd be wise to cut the box a bit bigger and deeper, and then cut shims to position your current phone so it's centered in the box.
Now imagine that the BACK side of the box is removable or hinged. That's the side you load the phone in.
On the front side, cut a front panel where the outside edges matches the box. Then cut an inside rectangle that aligns with the screen size (not phone size or internal dimension). What this does what the front is fixed to the box body is trap your phone against the front, while giving you access to the screen/surface).
Now make a second front panel, and hinge it to the first. The second front panel hides your screen when it's not in use. has steampunk greebles on it. has a latch to keep it closed.
that's the crux of the T-Case, you'd be doing something like this with the original pattern. I just don't fancy cleaning hundred year old grease.
