The main tool used in this instructable is the vertical mill. The project is not complicated, and would make an excellent first project on a mill.
Step 1: Make a Design
The exact shape of the handles, relative placement of the separating point and hinge, and size of magnet you're designing for are all up to you.
The first step is to cut your stock to size, and get out your tools.
I used two pieces of aluminum: one 1"x5", one 2"x5", both 0.5" thick. You could use a hard plastic instead if you prefer, but aluminum is cheap and strong and easy to work with, so I went with that. I've seen a similar design made of wood for large magnets, but for small magnets I worry the edge would wear out on something as soft as wood.
Step 2: First Cuts
Making the shoulder bolt work well requires that your stock be thinner than the shoulder. I used a fly cutter on the vertical mill to cut my stock down to a thickness of 0.498". This also produces an attractive shiny surface. The goal is to have the stock only slightly thinner than the length of the shoulder, so that there isn't a lot of room for movement. This produces a pleasantly smooth but not wobbly hinge.
(As you'll see in the pictures, I did this step out of order, but it really doesn't make much difference. It's just easier to get the slot depth right if you do your surfacing before cutting the slot.)
Step 3: The Base Block
The hole for the magnets should be slightly oversized. For 1/2" magnets, I used a 33/64" drill bit, with good results. 17/32" would probably also have worked just fine.
For the threaded hole, I used a tap wrench with a guide. The guide goes in the drill chuck, and holds the back end of the tap wrench. This ensures that the tap goes into the material straight instead of at an angle. Having the tap enter at an angle can produce angled threads (which won't work well with the shoulder bolt) or even break taps. This is especially a hazard in soft materials like aluminum. If you don't have a tap wrench like this, I highly recommend getting one. Failing that, be careful that your tap goes in straight! (Note that the mill is only used to hold the tap wrench guide; you turn the wrench by hand, and the mill spindle is off, for this process.)
Finish the base block with a modest countersink on the back side of the block; this will help make it easier to insert a stack of magnets. You can also countersink the threaded hole slightly; this is an easy way to deburr after tapping. Don't do too much here, or you'll lose the flat surface for the shoulder bolt to rest against.
Step 4: Build the Separator
For the hinge pin hole, I used a reamer to get exactly the right size. I drilled the hole to 31/64", then reamed it to 1/2". This results in a very smoothly moving hinge, without any play or wobble to it. I liked the feel of this hinge, and it was part of the reason for choosing the shoulder bolt design in the first place.
The slot for holding the separated magnet is also very simple. I used a 9/16" end mill to create enough clearance for easy operation. Just raise the work up to touch the end mill, raise it another 0.065" or so, and cut the slot. Make the slot a few thousandths of an inch deeper than the magnet, so that everything moves smoothly without jamming when the magnet separates.
Play with the separator a bit. See what works well. Don't be afraid to just take it to the vertical band saw and try something.
Have fun with your magnets!