How Do I Manually Select the Paper Tray?
When select your paper size for printing, many printers will automatically print from the first paper tray that matches that size, else from the manual feed tray. However, some printers can be rather...dense and always print from Tray #1 unless you tell it otherwise every single time.
This can obviously cause issues when you want to print from the envelope tray instead of the normal printer paper tray. Exactly how to do this will depend on your exact printer model, but you will need to be on the system print screen. So if you're printing from your browser's print screen, you'll need to scroll to the bottom and click "Print using the system dialog..."
This will bring up the system print dialog, where you'll need to hit "Preferences":
This will bring up a printer-dependent window, but typically there will be an obvious "Paper" option something like this:
Pick the correct tray where your envelopes are located, hit "OK", and you should be good to print!