gather your supplies in one clean workspace. I would suggest ironing your handkerchiefs prior to starting to save time. once that is done, you're ready to begin. Cut your cardstock into 4"x4" square.
Place your handkerchief over the square and determine the best section of the pattern of embroidery to use for your card.

cut out the section leaving atleast 1/2" extra fabric around designated embroidery or pattern area.

liberally apply matte medium to the back of the cardstock and place the handkerchief onto the card, right side up. place a heavy book over the card and let dry for about fifteen minutes.

after the card is dry, neatly trim the rest of the handkerchief edges.

use a corner rounder to round each of the 4 corners.

using an oversew stitch, neatly stitch around the perimeter of the card.

(hint: to ensure even stitches measure out the holes prior to starting a stitch.)

use the template which you can download here:
http://www.ohsosuite.com/handmade-and-special/2009/1/26/vintage-valentine.html

trace onto the piece of velum and cut out.

cover the velum in matte medium and press onto the second handkerchief. press firmly and push out all the wrinkles. (there will be a few you can't avoid.) cover with a book and let dry.

(option: you can forgo using the velum if you are not mailing. The velum adds sturdiness. But if you're just leaving on your valentine's desk, it's prettier without the velum.)

fold and crease along dotted lines. hot glue the bottom flap to the two side folds. If you're are just using the handkerchief, iron the folds and add a few small slip stitches to keep everything together.

write your love letter!