Try the following:
Take a half-sheet of photographic paper and place it in the important part of the photograph. Close the lens down 3 stops.
- Make a 5 second exposure;
- Cover up about a quarter of the paper with a sheet of cardboard and make another 5 second exposure;
- Move the cardboard to cover half the sheet and make a 10 second exposure;
- Move the cardboard to cover three quarters of the sheet and make a 20 second exposure.
This will give you a set of exposures at 1 stop intervals from 5 seconds to 40 seconds - ie 5, 10, 20 & 40 seconds.
If the whole sheet is too light then open up the lens 2 stops and make a series at 10, 10, 20, 40 second intervals for total times of 10, 20, 40, 80 seconds.
If the whole sheet is too dark then close down the lens 2 stops and make a series at 2, 2, 4, 8 second intervals for total times of 2, 4, 8, 16 seconds.
Either pick the best strip or make a new strip at some reasonable time intervals around your best guess. If, say, it looks like the best exposure is between 20 and 40 seconds then expose the paper for 20 seconds and cover it up by fifths making additional exposures of 5 seconds each, giving you exposures at 20, 25, 30, 35 and 40 seconds.
There is no particularly wrong way to make test strips.
You may want to adjust the aperture on the lens so your printing time is between 10 and 30 seconds. The best performance from your lens will generally be between 2 and 4 stops closed down from full open. If you have a very good lens the best performance may be at only 1 stop closed down. Aperture and time can be a trade off - for very small prints where there is a lot of light there isn't much harm from stopping the lens down 4 to 6 stops to get a reasonable printing time.
The most expedient and accurate results are from making strips at equal fractional-stop intervals. f-Stop timers have a feature that generates the correct times for making such test strips automatically.