First, the hardware: on the right 3 crochet hooks, of different sizes, 2,5mm, 2mm and 1,25mm.  The largest is appropriate for wool, the 2mm for standard cotton DMC, the small one for threads.  You can get them from weaving or needlepoints shops. Now to get started , we'll use a large fishing hook and 2 strands of knitting wool :

Place a large hook in the vice, tie on the shank side a 10cm (4") strand of  yellow wool then on the other side a 10cm (4") strand of  chestnut wool  (or any other contrasted colors). Put the strands well on the side and not on the top. Turn the vice so that the eye of the hook is in front of  you, the chestnut strand is on the right, the yellow on the left.

1-Take the chestnut strand,  make it go over the top of the shank and under the yellow.

2-Take the crochet hook, (here the 2,5mm). Slip it into the loop and below the shank.  The yellow strand is held by the left hand, make 2 turns around the crochet hook.  (a turn is insufficient, often the strand slips with traction).

3- Release the yellow strand and take the chestnut in the left hand.  With the hook, draw the yellow strand under the shank, through the loop.

4-Pull  completely, the yellow strand through the loop, lay the crochet hook, draw on the two strands at the same time, tighten the loop.

5-  The yellow strand is on the right, the chestnut on the left.  Lay the yellow under the hook and over the chestnut strand

6- Slip the crochet hook in the loop and over the top of the shank,  make two turns of the chestnut strand around the crochet hook.

7- Hold the yellow strand in the hand,   with the crochet hook, draw gently  the chestnut strand through the loop and over the shank

8- Release the end of the chestnut strand,  draw on the two strands and tighten the second loop.  The strands have returned to their initial positions, the chestnut on the right, the yellow on the left. For the third loop repeats steps 1 to 8, then again for the 4th, 5th

The finished body, seen from the top, the chestnut is in the center, the yellow on the edges

Seen from below , the yellow is in the center, the chestnut on the edges.  A common mistake is to swich  under/over during the process, for a loop the colors are inverted, in that case, go back, untighten and redo the loop.

It is not complicated, you'll catch the beat: under-over.  With this technique, you tie flat bodies, with colors variations , and beautiful solid flies.One can use various  materials, wools, cottons, strands of polyethylene...  In the weeks to come:  crocheted flies with dubbing, tinsel  and extended  bodies .  For enlarged pictures go next page.