Depending on your prescription, you have a couple of options using the viewfinder. One is to shoot while wearing your glasses. The other is too shoot without your glasses. In either case you are probably going to need to adjust your viewfinder's diopter setting.

Here's a picture of the diopter adjust wheel for your D90.

The trick is to find a comfortable position behind the viewfinder that you can consistently repeat. If you can do that with your glasses and see everything you need to see, great! Adjust the diopter setting according to the answers to How do I calibrate the diopter correction on my DSLR's viewfinder? You may have to shoot with it a few times to get comfortable.

If you find after several attempts that the glasses and viewfinder just don't work together, try shooting without them. If your prescription is within the range of -2 to +1 diopters, you should be able to adjust the viewfinder to correct for it. In effect, the viewfinder adjustment is taking the place of your prescription.

In either case, if you need a wider range of adjustment than that provided by the stock viewfinder, Nikon makes the DK-20C Correction Eyepiece for Rectangular-Style Viewfinder in ranges from -5 to +3 diopters. The stock viewfinder is centered at -1.0 diopters, so that is why there is no -1.0 diopter version of the DK-20C. The number on the adapters is the cumulative result of the stock viewfinder plus the adapter. So adding a -3.0 diopter adapter will center the viewfinder+adapter at -3.0 diopters, with an adjustable range of -4 to -1 diopters. Likewise, adding a +2 adapter will center the viewfinder+adapter at +2 diopters, with an adjustable range of +1 to +4 diopters.