My friend Rui from Brazil introduced me to lemongrass tea a few years ago. We were in the kitchen prepping lemongrass to make Tom Yum and Tom Ka soup. Lemongrass is so cheap in Thailand that cooks typically discard the top bits and use only the bottom, more fragrant, part of the stalk. My friend nearly passed out when he saw me throwing away the top part. He told me that those mild top stems are great for steeping tea.
Nowadays when I buy lemongrass, I clean the stalks and separate the top stems for making tea, and the bottom part for cooking. And I freeze them for later use.
My favorite way of making lemongrass tea is to add a bit of fresh lemon peel.
A good rule of thumb is to use 3-4 inches of lemongrass per each cup of water. I usually start out with 4 cups of water, with the corresponding amount of lemongrass. Bring the lemongrass and water to a boil, reduce to a simmer and add some lemon peel (I use about 1/2 a lemon or so). Allow it to steep on low heat until the flavors marry and concentrate to how you like it.
I prefer lemongrass tea on the gentler side (as opposed to ginger tea where I like it super strong).
When the tea is steeped to your liking, just add a little Turbinado sugar to sweeten at the end.