Add 20 g of premixed LB Agar, or 5 g Tryptone, 2.5 g yeast, 5 g NaCl and 7.5 g agar to the graduated cylinder. Add distilled water to a total volume of 500 mL. Mix this as thoroughly as possible and pour into the 1 liter Erlenmeyer flask.
Add a magnetic stir rod and place the flask on the stirring hot plate. Bring to a boil and stir for 1 minute to completely dissolve the powdered ingredients.
Cap the flask loosely with a piece of aluminum foil and label it with autoclave tape (lay the tape across the top of the foil and down two sides of the flask--about two inches, loose enough to allow steam to escape).
Use the oven mitts and place the flask in your autoclave pan with about half an inch of water in the bottom (per instructions for your machine). Autoclave for 20 minutes at the liquid setting.
Put on the oven mitts to remove the autoclaved flask of agar and let it cool to approximately 55 degrees C (to hand touch).
Prepare your Petri plates for pouring. Remove the sterile plates from their packaging, retaining the bag for later use. At the edge of the bottom of each plate, use a permanent marker to label them as "LB."
Pour the plates by lifting each lid just enough to pour in 10 to 15 mL of agar. Swirl gently to make sure the bottom of the plate is covered but not so much that you cause bubbles to form.
Let sit at least a half hour to fully cool.
Flip the plates over (agar side up) and place back into their original bag for refrigerated storage (label the bag "LB").