How do you meditate?

So I went to my doctor for my annual follow-up and she told me to read a book called The Diet Trap. I'm working through it, and I mean literally working because she didn't warn me it's a book you can just read, it's a workbook. It's based on something called Acceptance and Commitment Therapy (ACT) and so far it's lots of mindfulness, meditative exercises, like focusing on your breath or the environment.

My issues is that I can't seem to complete these because I keep falling asleep.

I'll be laying there, focusing on my breath, and next thing I know it's hours later. I'll get like five deep breaths in and BAM! I'm out of it.

How do people do this and stay awake?!?! Because I cannot figure it out.