Degree of spookiness can vary, especially based on glow colour. Does make a very good Halloween decoration.

Time: Some.

Skill: A little.

Caution: Unless you plan to sacrifice some dexterity by wearing work gloves, your hands will get scratched up. There might even be blood. Resign yourself to this fact now.

Step 1: Tools & Materials

Required

Wire. At least 0.5mm (24 gauge) thick, but better if 1mm (18 gauge). Anything below that will probably be too bendy, and anything too far over might be too tough.
Pliers, of both the grabbing and the cutting variety.
Glow powder
Clear epoxy
Small container
Stir stick, e.g. popsicle sticks or wooden skewers.

Recommended

Safety glasses. Really this should be in "Required" but hey, they're your eyes. If you want to risk 'em, have at it.
Needle-nose or round-nose pliers
Some kind of anchor point to loop wire around (wall hook, peg in a vice, arm of some hapless bystander...)

Optional

Long piece of wood
Clamps (F-clamps best, but C-clamps also functional).
Tape
Some kind of base on which to mount the finished tree.

Step 2: Wire Preparation Method #1

There are many ways to go from wire coiled on a spool to the many lengths of wire-in-a-bundle we need for our tree.

One method is to unwind the wire and cut off pieces of similar size as you go, then bunch them all together at the end.

Advantages are: a) small sections of wire are easier to handle than one huge one, especially if things need untangling, b) the end result is usually pretty straight from being stretched out to trim it, and won't be as eager to snap back to its previous coiled form, and c) if you know the total length of wire on the spool/in the pack, you can measure the pieces so everything comes out even with no big leftovers.

Disadvantages are: a) it takes a long time compared to other methods, b) if you have no idea of the total length of wire on the spool, you could end up under- or overshooting the size of each individual strand and end up with a trailing end too long to leave attached to another piece, but too short to properly incorporate into the tree, and c) you can end up running out of places to put newly cut strands where they won't get tangled with others, once you've already covered the tables and the chairs and the floor.

Step 3: Wire Preparation Method #2

Another method is to wind the wire around something, e.g. a piece of wood or maybe between two chair legs, that is close to the desired final length of the wire bundle. Or much longer, since the wire can be folded double and then double again, of course.

Main advantage here is it's way faster than the "cut everything as you go" approach.

Disadvantages are it can take a while to snip through the wires to get it all off the board, especially if it was wrapped very tightly; and the wire has a tendency to want to coil once it is removed from the board, and it takes a bit more effort to straighten a multi-strand collection of wire than to straighten a single piece.

Step 4: Wire Preparation Method #3

Yet another approach that is very similar to the previous method, except instead of wrapping wire around a rigid slab of wood, here we're using some F-clamps attached to a table.

Advantages are: it's way faster than clipping wire sections to length as you go; the adjustability of the clamps means they can be positioned anywhere for easy setting of custom wire wrap lengths; and the clamps can be just as easily loosened for very quick removal of the finished wire bundle.

Disadvantages are similar to the wood-slab-wrap in that mostly the wire wants to coil again and you have to fight it a little to make it not do that. Also, don't wrap the wire super tight, because if the clamps are not fastened tight enough and come loose partway through the wrapping process, welcome to Tangle City, population: You.

Step 5: Do the Twist

Now that all the wire is bundled up together, it's time to make it stay that way in the form of the tree trunk. Clamp one end down, but not all the way at the very end. Leave a bit of excess to turn into the roots/attach to a base later.

I used the hackerspace's lathe for this because hand-spinning* the chuck is an easy way to twist the wire, but if you don't have (access to) a lathe, mount the mess of wire in a vice and use either some pliers or just your bare hands to twist it. Don't twist the whole thing though. Branches still need making, after all. Leave around half of it untwisted.

*ABSOLUTELY DO NOT TURN THE LATHE ON AND TRY TO TWIST THE WIRE UNDER POWER. IT IS A BAD IDEA FOR MANY REASONS AND ALL OF THOSE REASONS END IN PAIN. LEAVE THE POWER DISCONNECTED AND DO THIS MANUALLY ONLY.

Step 6: Unloopy

The two ends of the wire bundle are probably still loops, depending on how the wire was all bunched together to begin with. Grab some cutters and snip open the loops on the end that will become the branches. The root loop can be left alone for now.

Step 7: Branching Out

Take the part that is going to become branches and separate it into between two and four sections (of course you can go more if you want, but I find 2-4 gives the best results), and twist each one of those the same way the trunk was. Sometimes it can be done entirely by hand, and sometimes it's easier using pliers to hold and twist the wire.

Don't bother trying to make each new clump of wire the same size. In fact, strive for unevenness. It will look more tree-like with variation.

Same as with making the trunk, don't twist all the way to the end. A few centimetres is good. Subdivide the wire cluster into a few smaller sections, and twist those. Keep subdividing and twisting until you reach the ends of the wires.

Step 8: Dealing With Loose Ends

Aren't those sharp ends annoying? Yes. Yes they are. You probably don't want to be scratched up each time you touch your tree, so it's time to do something about the ends. Grab some needle-nose or round-nose pliers and curl the ends in on themselves. If you're going to make the ends glow, you can be somewhat sloppy with the coils. If this is the end of the line for your tree, be a bit neater about it.

Step 9: Taking Root

Making roots is like making the branches, but less. The subdivide-twist-subdivide-twist is way less extreme, and the end result is flatter to serve as a base.

Step 10: Glowing "Leaves"

Want your spooky wire tree to be a spooky glowing wire tree? Grab some glow powder in your choice of colour, mix it with some clear epoxy, and dab it onto the curls made on the ends of the branches back in Step 8. This will make it pretty and glowy and considerably less stabby.

If you use a quick-setting epoxy similar to the type I used here, don't mix a huge batch of glow epoxy all at once. Make several small batches as needed, because a five-minute window of working time is not enough to get every branch done before the epoxy sets. You'll just end up wasting a lot of material if you try to do it all in one go.

I stuck my bottle cap mixing container to the table with duct tape so it didn't slide all over the place.

Step 11: Nix the Stabby-Stab

Curling over the ends of all the wires comprising the root system would just look weird, but even all bunched up as they are, they're still kind of sharp. Left this way they'll be prone to scratching things and getting snagged on fabric. A coating of epoxy over the wire ends will smooth it out. I used clear epoxy because that's what I had and it fits well with most things, but anything goes, really; add pigments to it if you want.

Another option is to attach the whole tree to a piece of wood or a rock or some other thing acting as a base. Choose your object, bend the roots around it in a way that looks all flowing and natural, and glue it in place.

Once everything is dry and cured and set, leave it out in some sunlight, or shine a UV lamp on it for a little while, and enjoy the glow.