The "bloomers" in the above photo are made with Caron Simply Soft yarn but any worsted weight will do. I used a size G hook. Claire (who is sporting my creation in the picture) is five-months-old and on the verge of moving into 6 to 9 month clothing so I'm going to say that's the size that this makes.
It is worked mainly in single crochet (sc) with double crochet (dc) shells. You need to know single crochet increases (2sc) and decreases (scd) and if you desire to sew up rather than slip stitch up the side, you'll want a yarn needle to finish off the project.
EDIT 7/4/12: Claire wore the pink one outside of the house yesterday for the first time and it got complimented by our neighbor, so I'm even more pleased with the piece. However, I noticed something about the second one I made: it's at least a full size bigger. I think this is because I worked tighter stitches using the Caron yarn than I did with the Red Heart yarn I used to construct the second. So keep that in mind when making your yarn choices.
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Chain (ch) 31, turn
Row (r) 1: Sc in the second chain from the hook, sc in the remaining chains, 30 stitches
R 2-3: Ch1, sc the length of the piece
Here we start to work the shells.
R4 (shell row): ch 3, 4dc in the first stitch of the row, (skip the next stitch, sc in next stitch, skip a stitch, 5dc in the next stitch
R5-7: ch 3, sc in the top (3rd) stitch of the shell, 5dc in the sc, repeat to the end
R8-13: ch 1, sc in each stitch across
R14-26: ch1, scd, work remaining stitches until the last two, scd again (each row will shrink by two stitches until you reach the desired width for the crotch. . .which I somehow neglected to count but I think it would be 18 stitches across at the end.
R27-40: ch1 sc across
R41-53 (I'm calling these my "increase rows, basically you are reversing the process you did in rows 14-26): ch1, 2sc in the first stitch, sc across until the last stitch, 2sc in the last stitch (you are increasing by two each row).
When you reach row 53 the width of the piece should be even with where it was in row 13.
R54-60: ch 1, sc across
R61-62: ch1, scd, sc across until the last two stitches, scd
R63: ch 1, sc across
R64 (A new shell row): ch 1, (single crochet in the first stitch, skip the next two stitches, 5dc (for a shell) in the next stitch, skip the next two stitches) repeat (-) until the last stitch, 3dc for a "half shell" in the last stitch.
R65-66: ch 1, sc in the top of the half shell, (5dc in the sc space, sc on the top <3rd stitch> of the next shell) repeat (-) until the last stitch, 3dc half shell in the last stitch
R67-68: ch 1, sc across.
Fasten off leaving a long tail for sewing
Here fold the piece together, matching up the edges carefully and slip stitch or sew the two edges together, leaving the leg holes open, of course. There. You're done!
This is how the piece looks when you've made the shell rows, the single crochet rows, and have started on the decrease rows.
This is just to show how the decreases look.
Somehow I neglected taking more pictures until I reached the second set of shells.
The piece when complete except for sewing up the edges.
Snug on her baby booty.