We have already learned the Backwards Loop Cast On method. That is a great technique to know if you need to cast on stitches in the middle of a project but it isn't generally the method you will want to use to start a project. It tends to create a loose cast on edge that is difficult to work with on the first row. It also does not stretch much… stretch is good especially on any type of cuffs (mittens, hats, sleeves, etc). I taught it first just because it is easier to teach and to get everyone knitting quickly.
Now I'm going to teach you a more versatile, more commonly used method of casting on called the Long Tail method. The Long Tail method creates a much nicer finished, stretchy edge. The trick here is just figuring out how long of a tail you need when you get started.
For this method of casting on, you need start by estimating how much yarn you will need to cast on all your stitches. For worsted weight yarn on size 8 needles 12 inches of yarn will give you approximately 20 stitches. Be sure to give yourself a little extra for a tail too.
There is nothing so frustrating as casting a couple hundred stitches for a blanket and realizing that you didn't give yourself a long enough tail and have to start all over. On the flip side… no one wants to waste yarn leaving a super long tail. You will get better at estimating tail length over time. One tip is to try wrapping the yarn around your needle and measuring how long of a strand of yarn it takes to make 20 wraps.
Once you have estimated how long of a tail you need, create a slip-knot (or you can just cast on the first stitch). Place it on your needle with the tail yarn closer to you, the live yarn in the back.
Hold the needle in your right hand. With your left hand, slip your index finger and thumb in between the yarn strands. Use your two or three other fingers to hold the yarn strands in place in your palm.
Spread out your thumb and index finger to make a Y shape. Tip the Y back slightly so you can see the yarns easier.
Slide the needle along your thumb under the yarn loop in an upward motion (the blue strand).
Reach the tip of the needle back and hook the yarn (from right to left) that is laying on the front of your index finger (the purple strand).
Slip that index finger yarn loop (purple), under the middle of the thumb loop (blue).
Slowly drop your thumb down and let that yarn fall free, creating a loop around the index yarn (the purple stitch).
Tighten your new stitch by tugging slightly on the yarn tail. Do not drop the yarn tails. Slip your thumb back between the yarn strands. Tip the Y shape back and you are ready to cast on another stitch.