Figured I'd show how i go about making agar plates. This recipe is about as simple as it can get and works effectively for most fungal cultures.
To start, measure 500ml warm water (cold will make the agar agar powder and the light malt extract clump together and not mix as effectively) in your chosen flask. I use a borosilica high-temp rated Erlenmeyer flask; some use whisky bottles, or even jars. Place a magnetic stir stick in the water.
Now, into the flask weigh out 10g LME (light malt extract) and 9g agar agar powder. You can do a 1:1 ratio of LME:agar agar or other variations. I find 10g of agar agar with this particular brand to harden up a bit too much once poured so I vary between 8g and 9g most of the time now. Add the two powders and set on your magnetic stirrer for a few minutes. I don't really measure this time as each batch is different, you just want to ensure the LME and agar agar powder are thoroughly mixed in the solution.
^ agar agar powder
^ Light Malt Extract. It's worth noting that LME is hygroscopic. If you even THINK about moisture at all, it will clump and get super sticky -- like kinetic sand that becomes glue.
Place your cap or cover on your container and tighten to closed position, then back off a 1/4 turn. This ensures that pressure build up doesn't turn your container into a bomb inside the PC. I cover mine with foil as a force of habit; not sure it's really necessary though.
Place your container in the PC and fill with water to the same height as the agar solution in your container. This is to help reduce the chances of boiling over as the solution cools off after sterilization.
^ Flask in water bath. The empty, no-lid jars are there to prevent the flask from tipping over while cooking.
Place the PC on your burner (if using one that requires external heat sources) and crank the heat as high as you can. You want to bring temps up as quickly as possible at this point so reduce the chances of caramelizing the sugars in the LME. Once you see steam coming out of the weight vent, set a timer for ten minutes. The idea is to purge all the air in the PC with steam.
When the 10m timer goes off, set your weight on the vent, and keep the heat up to get to 15psi as quickly as possible. Once the weight wobbles, reduce the heat and set a timer for 20 minutes. You want the heat to be high enough to maintain pressure but only so high to cause the weight to jiggle once or twice per minute. Any more than that and you're running too high of temperatures, and caramelizing sugars, any lower and you're not maintaining 15psi for adequate sterilization.
When the 20m timer goes off, shut off the heat. If you have a flow hood now is a good time to move the PC into that clean stream of air. As the PC cools, it creates a vacuum and has the potential of pulling in contaminates -- contaminates right into the sterile environment you just created.
Allow the PC to cool until the pressure lock drops, and carefully (using a silicon oven mitt) remove the container of agar solution from the cooker and set in your clean stream of air. At this point you should start monitoring the temperature of the agar solution with an infrared thermometer. Agar begins to solidify at approximately 122F (50C), and ideal pouring temperature is 131F (55C).
You will have to move quickly when the agar reaches that target temperature because the temperature really starts to drop at that point. I like to separate my stack of plates into two stacks of ten (500ml makes ~20 plates), and starting at the bottom of one stack, lift the lid and the entire stack, pour just enough to cover the entire bottom of the plate, replace the lid and move up the stack. With practice you'll get pretty good at this.
^ plates poured. The jars have hot water in them which helps prevent condensation from forming on the lids of the plates.
Allow the plates to cool for a few hours before wrapping with parafilm or inoculating with your chosen culture (spores, mycelium, fruit body, etc). Made plates can be stored in open air if wrapped with parafilm for up to a month, and even longer (I've kept some as long as six months) in refrigeration.
It's worth noting I did not cover additional sterile procedures. I assume if you're doing this, you know how to practice aseptic techniques and have all the appropriate equipment and supplies to successfully perform this process.
Happy mushrooming!