Materials:

• Round tape measure, a little over 2" (over 5cm) in diameter and about ½" (1.25cm) high, with the retracting button in the middle of one side.  I used Knit Picks' brand, but I believe Singer makes a nearly identical model.
• DK weight yarn, approximately 25yd (22m) for shell and 12yd (11m) for body.  I used Tahki Cotton Classic.
• Size E hook (3.5mm) or hook required for gauge.
• Small amount of fiberfill
• 2 6mm safety eyes (optional)
• Very small amount of red yarn or embroidery floss for mouth
• White craft glue
• Small amount of thin fabric (optional but recommended)
  

Gauge: In single crochet in the round, without increases or decreases.  24 stitches and 28 rows over 4" (10cm).  Gauge is very important so that the snail will fit the tape measure.  You can try to wing it, or you can make a small tube to get the gauge before starting.

All abbreviations are standard American crochet terms.

Instructions:

Back:
7sc in magic circle.
Row 1: inc around (14)
Row 2: (inc, sc) x 7 (21)
Row 3: (inc, 2sc) x 7 (28)
Row 4: (inc, 3sc) x 7 (35)
Row 5: (inc, 4sc) x 7 (42)
FO

Front:
6sc in magic circle
Row 1: inc around (12)
Row 2: (inc, sc) x 6 (18)
Row 3: (inc, 2sc) x 6 (24)
Row 4: (inc, 3sc) x 6 (30)
Row 5: (inc, 4sc) x 6 (36)
Row 6: (inc, 5sc) x 6 (42)
Row 7: sc around
Row 8: ch 3, trb in each sc except the last one (42)
FO, leaving a long tail to sew the front and back pieces together.

Body:
6sc in magic circle
Row 1: sc around
Row 2: (inc, 2sc) x 2 (8)
Row 3: sc around
Row 4:(inc, 3sc) x 2 (10)
Row 5: (2sc, inc, 2sc) x 2 (12)
Row 6: sc around
Row 7: (inc, 3sc) x 3 (15)
Row 8: sc around
Row 9: (inc, 4sc) x 3 (18)
Rows 10-16: sc around
Row 17: (sc, dec) x 6 (12)
Embroider on the mouth.  If using safety eyes, attach them now, or embroider on the eyes.  Stuff the body firmly enough so it will hold its shape but lightly enough that it will curve around the tape measure.
Row 18: dec around (6)
FO, sew the opening closed and leaving a long tail to sew the body onto the shell.

Antennae (x2):
ch 4, sk 1st chain and sc in ea rem ch (3sc).  FO, leaving several inches of tail on both ends.

Assembly:
Using whip stitch, start to sew the shell pieces together, stitch to stitch.  When you're about halfway done, place in the tape measure.  Make sure the retracting button is on the flat (back) side.  Line the slot where the tape measure comes out with the opening made when skipping the last trb.  I recommend pulling the tape out a bit to make it easier to keep track.  When you're about ¾ done, shove fiberfill into the domed (front) part of the shell.  Use enough so the shell will be very firm but also so you will still be able to finish sewing the front and back together.  The next part is optional, but I recommend that remove the fiberfill and then make a small, fabric pouch to hold it, putting the pouch in place.  This will keep wisps from coming out of the opening for the tape.  Finish sewing the pieces together, making sure for the final time that the tape comes through the opening.  Reinforce stitches on either side of the tape, then bury the yarn end.

Now, again with the tape pulled out a good bit, cut off the little plastic pull tab on the end.  Do not hit the retract button here or you will never see the tape end again!  Take one of the antennae and pass the ends through the loop at the end of the tape, one end going in each side.  Make little half-hitch knots on each edge and then pass each end through the again.  Place a small amount of white craft glue over each opening and allow to dry completely.  Do not retract yet!  After the glue is dry, carefully cut the yarn flush with the tape edge.

Once the glue is dry, retract the tape completely.  Place the body, lining up the head with the antenna pull tab so it looks good to you.  Securely sew the body onto the side of the shell, taking special care to put several stitches at the ends as they will pull the most.  Take the other antenna, line it up on the head so it's more or less symmetrical with the pull antenna, and sew that on.  And you're done!