If have like an SUV or some really heavy tankish car you might want to buy some pvc pipe(1" to 2" in diameter). I found it works better if it's warm/hot. First cut it down the pvc to the length of your sword. If it's a katana you need to bend it first, you can just do that with your hands. You'll need a friend who you trust or trusts you. Have one person on a driveway or whatever non-congested streeted area slowly drive the car on to the pvc pipe with the second person holding it in place. When you get to the place where the handle would be just call to stop the car. If it's a katana you may have to do it twice since it's not perfectly straight. You should at this point have a very flat piece of pvc piping.
If you don't have a jigsaw or bandsaw to cut the pointed tip, heat the pvc very very carefully over a gas flame until it yellows. Then with a strong pair of steel scissors snip off what you don't want. Then, even if you cut it with a saw, you'll want to heat the edges of the cut tip until it yellows and press the edges down with a stone rolling pin. When heating the pvc be careful that it doesn't start to catch fire or bubble. When it does that it creates a rough charred surface that you'd have to fill in.
For the handle use an air hardening clay like paper clay or plastiroc and add on all your detail for the handle. For a katana you might want to try using some balsa wood in conjunction with the clay. It's very important that you coat the parts you added on with resin afterwards. Then just sand the whole thing very lightly and paint it with whatever paints you want. Spray it with a finish and you got a shiny new sword.