Add all the marinade ingredients to a large Ziploc bag along with the pork pieces. Marinate for at least an hour, up to overnight (refrigerate).
If using wooden skewers, it's a good idea to soak them for 30 minutes beforehand to prevent excessive burning and splintering.
Preheat grill to high heat.
Thread the meat onto the skewers. Reduce heat to medium-high. Grill skewers for about 15 minutes total, turning every few minutes, or until the pork is cooked through. I cooked these with a gas BBQ with the lid down.
How to prep the lemongrass: Cut the dry ends off the stalk, then use your hands to peel the tough outer "shell" of the stalk. Discard that outer shell. Use a knife to slice the newly revealed inner stalk. You can then easily chop those slices up finely.
Fish sauce is quite aromatic, but it does mellow once you cook the pork.
Every grill/BBQ/grill pan is different, so please adjust cooking time as needed to ensure that the pork is cooked properly and to your liking. The USDA recommends cooking it to an internal temperature of 145F. Nowadays, slightly pink inside is generally considered to be ok (see this interesting article from Serious Eats that discusses how well pork should be cooked in more depth if you're interested in that sort of thing).