Ive been lurking on these forums for a while, but this is something that I can actually help someone with! So here goes:
Step One: Plan the weapon you are going to make.
Basically sketch it out and and figure out the rough dimensions that you are going to want it to be. Since you have a metal sword already you will be able to use that as a rough guideline and scale it down some so it will be more size appropriate for you.
Step Two: Shopping!
The basics that you will need for the project will be foam insulation (definitely get the pink or blue stuff, depends on if you go to home depot or lowes, the white styrofoam is just awful to work with and is actually pretty fragile) a broom handle (they have those at those stores as well) and paint. You will need two kinds. I suggest getting a regular old latex paint in black to use as a primer layer and basecoat because the second paint will be spray paint and if you dont basecoat the foam with the regular paint the chemicals in the spray will dissolve the foam and ruin your day. You are also going to need a styrofoam cutter. You can find them at Michaels or any craft store. http://www.kmart.com/shc/s/p_10151_1...0000000212587P
thats the one I use. Also pick up some liquid nails glue to join the foam together, it will create a strong bond without dissolving the foam in the process.
*some guidelines for buying the foam. The insulation is sold in a couple of different thicknesses. The size of the finished sword you want is going to determine which thickness you buy, and that should be pretty clear once you read the rest of this.
Step Three: Rough construction.
First off, if you havent used a foam cutter before go ahead and try it out now, unless you are going to make a massive sword, you are going to have some extra that you can practice on. Oh by the way make sure you have ventilation when you do the cutting because the cutter actually melts the foam and produces what I figure is not something you should be breathing.
The way Ive made my swords and the way that I suggest is a 3 piece construction. The Broom handle will form the core of the sword, and also give you a grip, that is surrounded by a sandwich of two pieces of foam. So sketch out your sword on the foam. Start with the core of the sword which is the flat part that each edge comes from in in the middle (this part is called the fuller, usually its a groove running most of the length of the blade but whether or not you make it a groove is up to you and whatever sword you are making), and then sketch out the edges up to and including the point. Do this twice on your piece of foam so what you end up with is two 2 dimensional pictures of the blade of your sword on the foam.
Using your cutter (which will be tricky at first I promise you but be patient) cut out the two pieces following the edges of the blade that you have drawn out.
Once you have your two blanks cut out flip them to the side you dont care about (i.e. if there are any scratched or imperfections) these will be the two sides that get glued together. Grab your broom handle and figure out how thick it is, now divide that in half and remember that number. Take the two blanks and figure out where the centers of them are and lay your broom handle on that line and trace around that, I suggest leaving a good couple of inches from the tip of the blade. Once you have done that (and I apologize that I dont have an easier way of doing this) take a sharp knife and cut into the foam in the outline of the broom handle, be careful not to cut too deep and go through the other side. With the outline scored just take a flathead screwdriver or any other flat strong tool and start chiseling out the cavity. Repeat for the other half. When you have the cavity for the handle made just test the fit of the broom handle and determine if you have to remove any more foam.
Once you have done that (I apologize for using the word "Once" so much) apply the liquid nails into the cavity for the handle and then put the handle into said cavity. Then apply a generous amount onto the the rest of the foam around the cavity, you can put some glue onto the exposed part of the broomstick if youd like, but liquid nails is some pretty tough stuff. Then carefully place the other half of the blade on top and press it firmly into place. With that done, get a bunch of heavy things and a towel or blanket that you really dont care about (weights, books, siblings, a lazy dog) and put them on top of the sword to apply pressure while the glue sets. Then, most importantly, TAKE A BREAK. The glue takes a long time to set up properly so I suggest leaving it overnight at the very least.
Step Four: Carving your sword.
Now is when the practicing with your foam cutter is going to pay off. Starting at the end opposite the tip (the one that should have a broom handle sticking out of it) line up the wire of the foam cutter so that one end is on the fuller line that you made and the other is right at the joint between the two pieces of foam. Then (with the cutter on obviously) slowly drag the cutter down the length of the blade until you come near the tip (this is going to be a different point for each individual sword so you will have to use your own judgement) and then pull the cutter out of the foam. Repeat for the other three cuts you have to make.
What you should have now is something that resembles a sword with a big club at the tip of it. The next step will be determined by the individual sword you are trying to make, so there is really no general step that I can give you other than take your cutter and use it to shape the tip to your preference.
*A little advice at this stage. You can use sandpaper to smooth out the cuts that you make but be careful with it as the foam is really unforgiving with sanding and gives you kind of a furry appearance, which is not very sword like in my opinion.
Step Five: Paint!
As I mentioned in the shopping step it is very important if you are going to use spray paint to give your sword a good basecoat with a latex paint. Because if you dont, when you spray it, the chemicals in the paint will dissolve the foam and ruin your day. I suggest using black because it really helps the colors you put on it to pop, and metallics will look more realistic.
Once your basecoat is dry, take some masking tape and mask off any parts you dont want a certain color and basically go to town painting your sword. Rustoleum makes some really nice metallic paints that actually look really good.
Step Six: Finishing your sword!
From this point on its really going to depend on what you are making as to how you finish. Obviously cut the broom handle down to the appropriate size for one. As far as materials for making the hilt and pommel and what not. You can use your cutter and the foam to make those and glue them in place before you paint. For projects I have done, I used Wonderflex (which you can source and find FAQs for using on this very site) to do all the decorative parts of my swords. At this point its basically up to you and your imagination as to how you finish your blade.
What you end up with is actually pretty durable thanks to the broom handle core and will stand up to the rigors of cosplaying, maybe not actual swordfighting though...
I just want to apologize for any typos in this very long post, as I was also playing wow and fighting with my cat while writing it. If I ever get around to it I will post pictures of things that I have done. If you have any questions feel free to PM me and I will help you out as best as I can.