You have two tasks, selecting the contact record you want for your envelope and then printing the contact's name and address on that envelope.
There are numerous ways to do the first task, you can perform finds manually, use a script, click a button on a list view, click a button in a portal....
The second task requires designing a layout where you place your fields on the layout in such a way that your printer can print it on the layout and this can differ with each printer. While in Layout mode, use printer setup to select your printer and select the paper size for the type of envelope that you plan to print on. This should then result in heavy lines appearing on your layout to show the size and orientation of your envelope so that you can correctly position your address fields on the envelope. You can enter preview mode to double check your results before printing.