Success with this will depend largely on how your envelopes feed. A
commercial printer will do this with the flap open, but that may not be
feasible for a desktop printer, and if you print with the flap closed, you
will have to be careful to keep the print area within the part of the flap
that doesn't overlap the rest of the envelope (so that the variation in
thickness doesn't distort the printing). Generally speaking, just select an
envelope paper size and position your text on it such that it will print in
the flap area.