The absolute easiest and quickest way to make an "amtgard
-legal" sword is to get a length of PVC (1/4") and a Funnoodle pool toy. The kind with the hole in the middle of it. Yeah. You also need two pennies and some amount of campfoam (or good foam cuttings skills).
A shortsword is up to 4' long in Amtgard, so for our purposes I'll go with this length. Cut the PVC pipe with a hacksaw or dremel (do this outside, it stinks!) to an inch shy of 4 feet long.
Next, you want to cut the foam so there is a blade along the majority of the PVC, a small handgrip (slightly larger than your hand is desireable), and another small (4-5") piece at the base of the weapon for protection and grip.
At the ends of both pieces of funnoodle you would be advised to duct tape the noodle to the PVC so it won't easily come off durring combat. Some glues have been reported to work well for this instead, your mileage may vary. Before doing this however, it is advisable to put a penny at the each end of the PVC pipe as to make it harder for the PVC to push through the foam.
Second to lastly, you cut campfoam into round disks and duct tape them to the ends of the sword. Two disks is plenty for the 'grip' end, and three is suggested for the 'bussiness' end.
Lastly, put two layers of stockings over the entire length of the sword and tie it off at the 'grip' end. These two layers not only make it look good, but also add to the lifespan of the weapon. I have weapons that I've used for two years with only minor repairs. I also advise using some kind of grip tape around the hand grip, the kind is up to you. I happen to get a good grip on duct tape, so I use it.
The most important thing to remember when making any boffer sword is to not use too much duct tape. If you must use tape for some reason other than securing something, use packing tape. It has the same amount of grip strength at far less than half the weight, and is also far more plyable.
This style of making weapons may sound very wussy-fied, and in comparison to Rattan-based weapons, it is, but it is safe enough for anyone that doesn't have a calcium deficiency to use and be hit with.
Flat Bladed swords are much harder to make but infintely better in the long run. For this you need:
a fiberglass (Wal-mart or Meijer, sold as property stakes or "snow guides". Long, orange/red, and $1.99.) or a graphite (old golf club) core. This will be the most expesive part.
A length of PVC equal to however long you want your hand grip.
Blue camp foam (the kind you sleep on).
Electrical or Duct tape (your choice, Electrical looks better IMO).
Cut the foam to length and width, 2 inches is a good width, and the length should be whatever is left over from the fiberglass after the hand grip. Leave a good inch extra at the tip for safety.
Next, spray-glue (if possible) both pieces of foam and sandwich the fiberglass between them. Make sure it's straight and to leave a good inch at the top! If you don't have spray glue, just forget it and packing tape the two pieces together with the rod between them. If you do it right, you can make some interesting sword shapes. Pay close attention to the tip, and make extra sure to put lots of tape there to ensure the rod won't poke through.
Next, put one or two pieces of camp foam (strips) into the PVC pipe and sandwich the fiberglass between them. It will take some work, but is quite do-able.
Make a pommel by putting a good bit of camp foam onto the end of the sword. Make sure you can't feel the PVC through it. The penny trick above would be a wise thing to do also. Again, tricks with using tape can make for some interesting effects.
Cover the sword with two legs of tights or something form-fitting along those lines (not panty-hose, but socks will work if you are creative), and end the cover by taping it to the grip for a nice smooth finish. This style of sword will last a bit less longer than the aforementioned one, but I assure you it will look better and hurt far less, especialy if you use fiberglass, as it has a slight whip to it and will bend easily if forced to. It requires more maintainence, but again, looks and general "ooh, pretty!" factor are well worth it.
After trying your hand at these weapons, make sure to stop by your local Amtgard group and have a jolly good time bashing people for no good reason!