I assumed that I could just pitch it after I broke the inner sac and let it expand. But the instructions say I have to make a starter. Is that necessary? Im brewing an ESB around 5.5% alcohol.
You do not have to make a starter unless the yeast is old. If you can you should. It is easy and you will start fermenting a lot faster, plus, you will know that the yeast is living before you brew.
If you have the 125ml pack, a starter is not necessary for beers with an O.G. of 1.065 or under. I can say from personal experience that I've had excellent results and short lag times when I aerate well and pitch drectly from the swollen smack pack into 5 gallons of wort. Never hurts to make a starter though.
You won't need luck, the hard part is patience.
I recommend starting another batch while this one is fermenting so you wont be tempted to meddle with the first batch.