What should I set the Canon XSi in terms of flash settings? Also what would you recommend for ISO and aperture settings when using this off-camera flash (that doesn't synch with camera)? I have tried manual setting on flash.
Manual setting is usually the right way to go with the flash. What you set your camera on depends mainly on whether you are balancing flash with ambient light and how much output you are willing and able to get out of the flash.
If you're using flash only (no ambient light) the shutter is unimportant so long as it doesn't exceed the sync speed (maximum shutter speed in which the whole frame is exposed at once) this typically means keeping it slower than 1/200s. I usually set the aperture at f/5.6 for maximum sharpness, though you might need to stop down for DOF. ISO as low as possible (100-400). If your flash output is limited or you want to conserve battery power, up the ISO or open the aperture.
If you are balancing flash with ambient light this will dictate your shutter speed, set the shutter to get a good balance then set the aperture and ISO as above to gain the correct exposure.
If you are syncing by hand (not electronically) you will need very low ambient light levels, a shutter speed of around 1/5 - 1/10s, and good reflexes!
The answer depends on the effect you want to achieve. First you need to workeout how to set up your flash based on distance from object to flash and camera f-stop. You will need a table to allow you to work out how to set up your flash (such as this one - there may be something similar on your flash unit) based on the following information:
Then you can just expose manually for the unlit parts of your scene. If you want a black background, stop the lens right down, but use a long enough shutter speed that will allow you to fire the flash while the shutter is open.