Gather your supplies:
4"x4" cardstock - atleast 100lb
2 vintage handkerchiefs - preferably with embroidery
Envelope Template
1 sheet, 8.5"x11 velum paper
matte medium
sponge brush
corner rounder
hot glue gun
hot Iron
And as usual - "the basics" (pencil, ruler, scissors, cutting matte)

Set up your clean workspace and iron your handkerchiefs. Have your cardstock cut into a 4"x4" square and determine what pattern from your handkerchief you want to use.

Step 1: Layout

Cut out section of handkerchief after laying over cardstock and determining best use of design. Be sure to leave a1/2" or so of extra fabric.

Step 2: Adhere

Liberally apply matte medium to cardstock and place handkerchief onto card, right side up. Place a heavy book over the card and let dry completely - about 30 minutes.

Step 3: Dry and Trim

After the card is dry, neatly trim the edges.

Step 4: Round Corners

Use a corner rounder and round each of the 4 corners.

Step 5: Stitch Detail

use an oversew stitch and neatly stitch around the perimeter of the card.
HINT: to ensure even stitches measure out holes prior to starting a stitch.

Step 6: Envelope

Use the template and trace onto velum and cut out.

Step 7: Velum for Mailing

cover the velum in matte medium and press onto second handkerchief. press firmly and push out all the air bubbles. there will be a few, you can't avoid. Cover with a heavy book and let dry.

OPTION: You can forgo using velum if you're not mailing. It just adds the stiffness you'll need to send through USPS. But if you'll just be dropping off on your Valentine's desk, then I'd say skip the velum as it looks prettier with just the hankie.

Step 8: Finish Envelope

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