All you need to construct a "toy" lightsaber is a plastic tube illuminated by a bright interior bulb. Some are made from telescoping sections that can collapse into the handle when not turned on.
An actual lightsaber is not physically possible to build at present, because it requires light energy to be both coherent and contained in a field. Projection of an unsupported magnetic field is complicated by the necessary field strength, which would affect a lot more than the energy within the "saber."
You should be asking the "what?" before you ask the "how?" In the sci-fi terms that define what a lightsaber is constructed of, it wouldn't work. But even George Lucas himself admitted that he was simply creating fiction and didn't know anything about how it would work. He just called it a lightsaber because...well...it emits light. When you go home and say "turn on the light, would ya?" are you referring to a nonstop beam of light? No, you're visually describing an incandescent bulb operated by electricity, which of course emits light. Same thing applies to a lightsaber - it emits light, but that doesn't mean it is made of light. When little Anakin told Qui Gon that he saw his laser sword, are you going to accept that as fact? Anakin was a kid when he said that, and had no knowledge of how a lightsaber was constructed, nor physics for that matter. You're gonna go off the word of a KID?
What? If you really want to build a lightsaber, you have no choice but to go with existing technology, and if it doesn't exist, well then - invent it. A plasma saber can be called a lightsaber because it would emit light. Plasma is possible here people. It occurs naturally everyday. Physicists always yammer about how you can't generate enough power to inject a plasma into a magnetic field, which bugs me because they're always talking about plasma and magnetic fields as if one has absolutely nothing to do with the other - like they should be regarded as two separate and unlinked entities. Let's talk electricity. A flow of charge generates an electromagnetic field, and vice-versa an EM field will generate a flow of charge. Now let's talk about conductivity. Your best conductors are silver and copper. These are good conductors. Some materials oppose a flow of charge - these are known as nonconductors. But, as fate would have it, nonconductors will conduct just fine if they become ionized. For example, a simple capacitor utilizes this concept. A charge builds up one one end of the capacitor, seeking to dump to the other end to complete its circuit. To accomplish this task, a nonconductor is sandwiched in between. And that's when something amazing happens. When enough charge builds up, the nonconductor becomes ionized and the capacitor is able to make its charge dump. When this occurs, the nonconductive material returns to its nonconductive state and the process starts all over again.
How? Of course, you're thinking, how does this answer my question? We live on a giant capacitor. The clouds in the sky will gradually build up a static charge. And naturally, when enough charge builds up, you get lightning. Air is a nonconductive material until it becomes ionized. And when a material becomes ionized, this means that each atom has enough energy to excite its electrons to another level until one gets bumped off. What results is a flow of charge, a flow of electrons, charge in motion - a current, pushed by a massive voltage. This of course leads me right into plasma. What IS plasma? Plasma is simply an ionized gas. It doesn't matter what type of gas it is, as long as it IS a gas. Air is a gas, and when it becomes ionized, it is converted to plasma - this is what we call lightning. And, of course, I'll say it again - any charge in motion generates an EM field, and therefore plasmas generate their own EM fields. This is all I will say for now - that lightning (electron arcing) is the simplest way to create a plasma saber. Of course, lightning zigzags, because it follows a nonlinear path of the least resistive molecules in the air. If you use a laser in tandem to heat the molecules, this will provide a linear path of least resistance. And PLEASE remember that like polarities will always repel. If you keep it simple, simple solutions will always present themselves.