Hmmm...I never used a camera with a manual aperture setting, so it would probably be better for someone else to handle this.
In case no one else chimes in, I'll guess based on the way my teleconverter works. When I connect mine, everything still works the same, but the minimum f-stop has changed to two steps higher. The f-stop number is the ratio of the aperture to the focal length. Presumably the aperture hasn't changed, but the focal length has doubled. So, I would guess that when you set the f-stop on the lens, it works as though it was two stops greater.
So your f/8 1/60 ISO 400 would be f/16 1/60 ISO 400. Because the light level hasn't changed, that would lead to a two stop underexposure. You could correct that by opening your aperture two stops (so that it says f/4 but is effectively f/8 again) or you could cut your shutter speed to 1/15 or you could change your ISO to 1600 or some combination of changes that has the same result.
Since this is all manual, I'm assuming that you are using film. In that case, you can't really change your ISO. You can change the setting on the camera, but if it doesn't match the film, it won't give you the results that you want. Since I assume that you won't be changing film when you put on or take off your teleconverter, you'll probably want to adjust the f-stop or the shutter speed.
If I'm right (and I'm writing this based on theory, not real-world experience), you want to either decrease your f-stop two stops, decrease your shutter speed two stops, or decrease both of them one stop. On the scales below, decreasing by one stop means moving up the scale one line.
F-Stops
f/1.0
f/1.4
f/2
f/2.8
f/4
f/5.6
f/8
f/11
f/16
f/22
f/32
Shutter Speeds
1
1/2
1/4
1/8
1/15
1/30
1/60
1/125
1/250
1/500
1/1000
1/2000
1/4000
1/8000
So if you put your teleconverter, find your shutter speed and aperture on the charts above and then move up two lines (either two lines on one chart or one line on each).
Now, if your camera has a built in meter, it should, I would think, compensate automatically for having the teleconverter on the camera. In that case, don't worry about conversions and just set the settings so that you get a proper exposure. The way to test that is to set your camera to a proper exposure without the teleconverter. Then add the teleconverter and set it to the proper exposure. Then compare the exposures. If they are the same, you need to compensate using the chart above. If they are different, they should be different by the amount of the chart. In that case, the camera's meter is doing the work and you needn't worry about a chart.
Of course, it would be much easier to upgrade to an automatic camera.
Since it's metering inside the camera body, it should be doing that through the lens so it's taking the light loss into account (not unlike putting a dark filter on the front of your lens.) So I think you'd just use the camera normally, don't think too hard about the difference, but you will be ending up setting your exposures two steps higher.
