Front side

You should have a slip or a paper cut pattern to compare it with your crochet slip.

1) Slipknot
2) Chain stitches (width of your slip)
3) Chain 2 more stitches (this will be the first double crochet)
4) Yarn over, insert your hook into the 4th stitch from hook, double crochet all stitches
5) 2nd row: chain 2, turn your work, double crochet
6) 3rd row: chain 2, turn your work, double crochet
7) Cut your yarn, bind off (strand should be long enough for sewing later)

8) Put your crochet strap on your slip and set markers where to crochet the triangle (watch video how to do http://youtu.be/92jnfw83-hY?t=6m50s) Count stitches on each side to make sure it is symmetric, if not move your markers to have the same number of stitches on each side.
9) Tie your yarn strand next to the first stitch where the first marker is located. (http://youtu.be/92jnfw83-hY?t=10m4s)
10) Insert hook into the stitch where your first marker is located. Slip stitch. Chain 1 stitch.
11) Double crochet until the second marker.
12) 2nd row: chain 2, turn your work, decrease once: 2 double crochets become one (http://youtu.be/92jnfw83-hY?t=12m1s), at the end of the row you should have 3 stitches left, decrease once, double crochet.
13) Crochet so many rows with decreases as you need (compare with your slip or pattern). I have 3 rows.

14) Chain 2, turn your work, insert hook into the 4th stitch from hook, double crochet
15) Crochet so many rows as you need until you reach the crotch area (compare with your slip or pattern).

16) Chain 1, turn your work, yarn over, insert hook into 2nd chain from hook, half double crochet until the end of the row.
17) 2nd row: chain 1, turn your work, yarn over, insert hook into 2nd chain from hook, half double crochet until the end of the row.
18) Crochet so many rows until you finish the front side.

Back side

1) Chain 2, turn your work, increase once: 2 double crochets into the same stitch (http://youtu.be/-01KDq8tBGE?t=34s), double crochet until the end of the row, increase once a the end of the row.
2) Crochet so many rows (increasing) until you reach the strap region from the beginning.
3) Double crochet the last rows without increasing, bind off (cut the strand to have enough length for sewing)

4) Sew or slip stitch together your left and right slip side (http://youtu.be/-01KDq8tBGE?t=6m10s)