The backwards loop cast on is the simplest of cast ons, but it is also the least structural. For this reason, we don't recommend it except for very short stretches of cast on stitches.
Step-by-Step written instructions with pictures:
Set-up: make a slip knot close to the end of your yarn and put it on your needle tip.
Wrap the yarn around your finger and place it "backwards" onto the needle tip, capturing the live strand between the two loops. Do this as many times as you need.
The reason we don't recommend it for casting on many stitches is that this cast on has a tendency to stretch out as you knit the first row, so it doesn't have the structural integrity of the other short tail cast ons like the knitted cast on or double-loop short tail cast on.