A step wedge is a strip of film which has been manufactured so that a series of different densities is represented. You can buy them calibrated ($$$) or uncalibrated (~$9) but the uncalibrated ones work just fine. The density of step one is about 0.05 and each step progresses by about one-half stop per step. So, step 2 can be taken to be 0.20, step 3 @ 0.35 and so on until you reach step 21. Now a proper van dyke brown emulsion might print somewhere between 16-18 total steps which would represent about an 8-9 stop total exposure range (with corresponding densities of 2.40 to 2.60) depending on your emulsion mix and specific paper.
To provide an example: step 1 should print out as the maximum density the process is capable of while perhaps step 16 would be paper white and step 15 just the lightest of gray tones. Steps 16-21 would all be white. If you increased the exposure the entire range would shift with step 1&2 maximum density but now step 16 is light gray and 17-21 paper white. This would indicate an overexposure of one step which is equal to one-half stop. You want to adjust exposure to get the maximum number of steps printed without blocking up and at the same time having the maximum density possible. This range will vary with your paper and emulsion mix.
The true emulsion exposure range is actually pretty easy to determine with the step wedge. The tough part is adjusting your image negative exposure and processing so that it matches the contrast (exposure range) of the process you are working with. VDB is fairly high while Pt/Pd is less and normal silver printing below that. I'd rank the processes from lowest to highest negative contrast needed as: gum bichromate, silver, cyanotype, Pt/Pd, VDB, Albumen, POP, salted paper. A negative made for van dyke brownprinting really won't make a good silverprint even if a contrast #0 filter or paper is used.
I have a few web pages showing VDB exposure test using step wedges. The urls are:
http://my.net-link.net/~jsmigiel/images/technical/VDB_TMAX.jpg
and
http://my.net-link.net/~jsmigiel/images/technical/TMX_TMY_HP5.jpg
These tests were done because it was discovered new TMAX 100 has an UV blocking base which makes it a poor choice for printing in alternative processes like VDB. The tests compare exposures through new and old TMAX films, HP5+, and just the step wedge on paper. They also show the effect of processing and drydown with vdb (the difference between the marks for exposure [EXP] and dry print).
As far as the exposure source, pick up some UV black light tubes that fit standard 24" fluorescent fixtures but be aware that the UV can damge your eyes so don't watch the exposure process. The Sun will also work but since it is not consistent this time of year you will need to use one of the hinged frames to check the progress of exposure. VDB is fairly fast so I doubt your exposures will be anywhere near six hours midday. My exposures with the NuArc are about 20 minutes IIRC and, 6" from UV tubes even faster. (But, the NuArc is preferred because it is so consistent and eliminates the guesswork and monitoring.)