Type the sender and recipient addresses, choose your envelope size and font size, and nudge the offsets if your printer runs off-centre. Print a test on paper first.
It depends on the printer, and the only reliable method is a test: mark one corner of a blank envelope, feed it, and see where the text lands. Most desktop printers take envelopes face up with the flap to the left, but manufacturers differ.
Use the horizontal and vertical offset controls to shift both blocks together, and make sure your print dialog is set to 100% scale with the envelope size selected as the paper. Scaling set to 'fit to page' will always misplace it.
DL (110 x 220 mm), C5 (162 x 229 mm), C6 (114 x 162 mm) and US #10 (4.125 x 9.5 in), covering the common business sizes in Europe, India and North America. The preview shows the true proportions of the one you pick.