Take any size envelope that larger ones are better. Glue the flap shut with extra adhesive. Then cut a small piece off one of the ends, just to create an opening. Now score at 1/4" or 1/2" depending on how large of a side gusset you want. Score the same on all three sides. Do not score on the side with the opening. Reinforce your score lines and burnish.
Now open the bag and put your had in using your fingers to push out and create the bottom. You will end up with two ear like pieces on either end, these will need to be glued down with a strong adhesive. This creates the bottom of the bag.
On the sides push in on the center score mark creating the gusset.
Now on the top where you cut the piece off to create the opening in the envelope, measure down 3/4" and draw a line. Then from either side measure in on that line you just drew and make a mark at 1 1/4". Cut on the line that goes across just up to the 1 1/4" marks you made. Do this on both sides. You are creating the part for the collar. Fold down each piece to create your collar and glue together. Don't glue down on the shirt part until you put your tie on.