I believed there are many ways of finishing a Mail Art. I don't claim this to be my method, only that I find this way to my convenient as I don't need a sewing machine to finished this.
First determine the size of your envelope.
Complete your stitching first. Make sure your stitched design is correctly placed.
Shown here is Aida 16ct approximately 19.5cm(W) and 33cm (L) including allowance for seams.
There's no standard size for Mail Art but I find this the most comfortable size to worked with. My Mail Art varied in sizes according to my stitch design. Also, your mail art must not be too small. Afterall its going into the mail system or should fit in your envelope if you are mailing it in an envelope. So do take these into considerations.
I apologize for not revealing the face of this MA for this is actually for an exchange and I never really got to finish it off yet but its possible to show the finishing method.
I iron my interfacing on to the lining fabric (on wrong side) and not the stitched fabric, for fear of possible staining .
Place right sides together, the stitched piece plus the lining fabric after iron-on and mark approximately 1cm from edge and sew running stitch along the Width (W) which will form part
(B) in the diagram below.
NOTE : Before sewing, determine the back fold/pocket where the slot will be for this part is to from the pocket edging for the back
Spread the sewed two pieces of fabric apart, flat on the surface with the lining piece marked L (b) on the lower part (nearer you) to work on W2 (when you get a hang of this, you need not follow this to the T)
Pressed iron over on the sewed seams downward towards you (B) with a 1cm fold
Now bring and fold the lining fabric marked L(b) over to the stitched piece, pressing along the seam with both wrong sidenow facing each other in which it should looked like below (Giving the edge now a bias like look ) (Hope I make sense here)

You will note that now the stitched piece is longer than the lining piece. No fear.
Its meant to look like that. Now pressed/fold 1/4 length (or your desired pocket length/depth) of the fabric to form the pocket.
I fold approximate 11cm out of my 33cm length which should look like below
Next, marked the side seams as shown below approximate 1 cm on each sides (you may want to pin the fabrics down at this point) Pin any where you felt threated it will fall apart, LOL
This is where you will worked the side seams, fold or by tucking in approximately 1cm on both sides as marked above.

For sides seams closures, tucked in approximately 1cm on both sides, working only one side at a time
You will note that were the pocket area is, will now have four layers
The stitched piece and lining (sandwiching the iron on interfacing) from the pocket's end plus 2 more layers (in between sandwiching the iron on) from where the frontal of the stitched piece.
From along where that pocket area is, you will need to fold/tuck in 3 layers from; the back pocket stitching fabric plus 2 lining layers leaving only one layer which makes the stitched piece
Note : FOLD only this 3 layers leaving the stitched piece facing frontal folded as it is.
Pic above : Lift and fold 1 cm inward from where the pocket is, leaving only the outer layer of the stitched piece which will be folded as a single layer.
Pic below : Fold and s
mooth the corner and the edge of the Bias look-alike (B) with fingers to keep it from creasing.
Make a smooth tuck in to complete the pocket section. The section above the pocket (closure) will be only 2 layers, the lining fabric (plus the iron-on) and the stitched piece. Confuse ? I hope the photos below will give you a better view

View : On your Left is the upper section flap envelope closure ----> right is where the pocket after fold is. Note that the fabric is visible on the insdie of the front flap.

Before heming the edge, this is how it should look like after the 1cm fold /tuck-ins all round . What you see facing you is the
back where the flap/cover and the pocket (take note how the edge of Bias (B) of the envelope is folded.
Note the outer layer which is the stitched piece of the envelope with seams folded in as it is
Also, on your left is where the frontal or flap cover with a very visible fabric lining on the inside of the envelope
Press and pin or running stitch before heming the seams up with any stitch ( Hems, Banket stitch, Herringbone ) you are comfortable with.
This is the part to add laces or ric-rac edging if you want a little decorated side edging.
There is no correct way or wrong way to sew as long as you seams it. You can also use the sewing machine here.
A closeup on how the edge/side should look like
I leave you with some pictures to show the side heming/sewing in progress
Repeat for the other side
Remember to add the edging if you had done so on the opposite end. Once done, you will have a pocket like below
To work the closure or the flap of the envelope.
You will note that my envelope size leaves you with approximately 2 cm longer on the stitched piece from the lining fabric. Fold that 1 cm, iron and press or simply add laces or ribbons to complete the edging.

If ribbon is intended to close the envelope, cut a length of approximate 24cm, or any length you are comfortable with, fold half and attached to the centre of the envelope as indicated by the arrow. make another 1cm fold after and hem it and with that make another fold of about 5cm more or less from here to create the envelope flap.

You may have two ribbons or more, buttons or velcro to secure the flap. Put a tiny stitch to hold the ribbon in place before heming the flap edge. Laces or ribbons may be added to finishing that top seam.

To secure the flap, mark the centre for my case, just below the closure or one or two even three cm away and sew a button for the ribbon to tie on . Add any embellishment to decorate your envelope You can finish the flap or the length size anyway you desire
AND DONE !
This is how the Mail Art should look like with the above sewing method.

The
front ? The front is considered simply because that is where the address of the recipient is
The
Back is so identify because it is where the address of the sender is.
BUT we know that it is not necessary on Mail Art envelope. It can be just the opposite.
Thank you for taking a peek. I hope my tutorial make sense. LOL, enjoy .