A quick and easy way to get your exposure is to use the guide number, distance and f/stop method.
The guide number of that flash is roughly 110 ft at ISO 100 at 35 mm setting. Divide the guide number by the distance between the flash and the camera for the correct f/stop.
For example, you are taking a picture of a person who is 15 feet away from the flash (not the camera unless the flash is on the camera). Divide 110/15 = 7.3. Your correct f/stop will then be f/7.3 or you can round up to f/8.
Take a picture and review your shot/histogram and adjust your f/stop as needed.
You can rework the equation to fit the situation. If you want to shoot at a particular f/stop, and you know the guide number, then you can calculate how far away your flash should be as follows: d=GN/f. So if you wanted to use the GN 110, and you want to shoot at f/8, you should place your camera 13.75 feet away from your subject because d=110/8=13.75.
The thing you need to remember, is that the GN of your flash changes based on the zoom head setting of the flash. The 110 ft GN is good for 35 MM, but the GN will be higher if your flash is set to 50mm or 85 mm. You can look these figures up online by conducting a search for SB-28 guide number, and create a little chart you can print on an index card and keep in your kit.
Something to keep in mind is that this does not need to be precise. If I am in the field and have to shoot in manual mode, I tend to ballpark the math. In other words, I would use f/8 instead of f/7.3, and I would use 15 feet instead of 13.75 as calculated above. You just need to get it close, then review your image and histogram and make any fine adjustments you deem necessary.
Of course, if you want to cheat, you could buy a flash meter.
In reality is is a lot more complicated to explain than to perform once you get the concept. Good luck!