Materials:
4.5mm hook
Worsted weight yarn - Black, white, red, two similar (or contrasting) colors for the iris & a small amount of orange.
Crochet thread or embroidery floss in red.
Fiber fill, scissors, yarn needle, stitch marker

Abbreviations used:
MR - magic ring SC - single crochet INC - increase
DEC - decrease SLST - slip stitch FO - fasten off
ST - stitch BLO - back loop only

Gauge is not important
The entire octopus takes less than one skein of yarn.
Finished octopus sits approximately 6 inches high, each tentacle is about 8 inches long.
A smaller hook, will make a smaller finished octopus, a larger hook will make him bigger.
I like to use the 4.5mm hook, because it is relatively easy to keep the stitches tight enough so that there are no holes for the stuffing to show through.