You mix it with water, soy sauce, and seasoning and it becomes an elastic ball.  If you simmer it and have to cut it into pieces, kitchen shears work better than a knife.  When you simmer it, you also use seasoning in the water.  Recipes will tell you the amounts.  If you bake it, you mix the vital wheat gluten together with water and seasoning, then wrap it in foil and bake it in the oven.

I really like this simmered chicken seitan recipe from all recipes:

http://allrecipes.com/Recipe/Chicken-Seitan/Detail.aspx

Or you can bake it:

Here's my re-post of a great way to "bake" seitan.  I've been using this recipe from Vegan Vittles for years:

From Vegan Vittles: Seitan Salami, Pepperonni & Pastrami,

Dry Ingredients
1 1/2 cups vital wheat gluten
1/4 cup nutritional yeast (I've left it out and it's fine with out it)

Salami & Pepperoni Seasaonings:
1 Tbl. paprika
1/2 t. ground cumin
1/2 t. of dry mustard
1/4 t. onion granules
1/4 t. ground black pepper
1/8 t. cayenne pepper

Pastrami Seasonings:
2 Tbl.paprika
1/4 t. ground cinnamon
1/4 t. ground cumin
1/4 t. black pepper
1/8 t. cayenne pepper
1/8 t. ground allspice

Liquid Ingredients:
3/4 cup water
4 Tbl. tomato paste
2 Tbl. soy sauce
2 Tbl. olive oil (I double that amount)
3 cloves garlic pressed
1 t. salt

Place the gluten, NY, and the seasonings you are using in a large mixing bowl and stir.

Place the water, tomato paste, soy sauce, olive oil, and salt in a med. mixing bowl and whisk together. Pour into dry mixture and mix thoughly. If there is still flour around the edges mix a "tiny" amount of water (drips).

Now knead the gluten in the mixing bowl for a minute.

Form the gluten into one long log. Wrap the log in tin foil tightly and twist the ends.

Bake at 350.
Salami and Pastrami for 1 1/2 hours
Pepperoni for 60 minutes.

Unwrap the seitan and transfer to a cooling rack. When completey cool wrap tightly in plastic wrap and chill for several hours before eating

Have a few beers, glasses of wine, shots, or any other cocktail of choice while waiting (my own additional advice).
ENJOY!
-dave