Kellee,
The paper orientation thing sure can be confusing. The way I think of it, the paper normally will go through your printer short side first. Then, if you want it to print so you can read it standing in front of the printer, the Page Setup orientation should be set to Portrait. If you want it to print so you can read it standing to the side of the printer, the orientation should be set to Landscape.
Now, I think you are asking how to set the orientation so that your print orientation is correct on the envelope if you feed it long side first. If so, use Portrait orientation and a custom paper size that has the long and short dimensions reversed.
Best to experiment with some plain paper before sticking an envelope in the printer. But, even before that you can select a low zoom magnification so that the entire envelope is shown while you are editing and see if it looks right.
Jerry