Mac (works): Print, then choose paper size Envelope #10. Feed the envelope the way the symbol on the printer shows.
Printing from a phone has known issues. iOS gives no paper-size control, so it fits the page onto Letter and can come out shifted or upside down. Letter sheet mode under Layout is an untested attempt at a fix. Until it is verified, print from the Mac.
Layout
Emits an 8.5″ × 11″ portrait PDF with the addresses rotated and placed so they land right when the printer images a Letter page onto an envelope fed short-edge-first. Because the page is already portrait, iOS has nothing to auto-rotate. The two unknowns below need a test print to settle.
Default assumes the slot centre-registers a 4.125″ envelope in an 8.5″ width, hence 2.1875″. If the print lands left or right of centre, change that number by the amount it missed by. If it prints upside down, tick the flip.
Everything here is saved on this device, so the return address only gets typed once.
Feeding the Brother
1. Open the back cover and pull down the two green levers, one each side — straight paper path, no curl. Put them back up afterward or plain paper will jam.
2. Print settings: paper #10 Envelope, media type Envelopes (Env. Thick if it smears), margins None, scale 100%, headers & footers off.
3. Front manual feed slot: guides to the envelope width, feed one envelope — print side up, flap up, top edge first. Push until the roller grabs, then let go.
4. Sideways result? Tick Rotate 90° instead of fighting the driver.