Gotta read that box before you put down the ducats, sonny.

Some people just boil instead of pressure cooking and get good results so I'm sure that if you are a little careful about mixing things (trying to keep it as clean as possible) and make sure that the substrait you are using is easy to sterilize you'll be OK.
My advice would be to keep the contams you add while mixing to a minimum so you are relying less on the canner killing everything, making sure you have NO sunflower seeds in whatever you use as a substrait and cook it about 10 minutes longer and you should be OK.
If things go wrong the first time dont feel to bad, this can be tricky sometimes but you CAN do it.

Shaker

tekniq

Adaption 23 in the Magic Mushroom Growers Guide (available in the grow find section) talks about sterilizing substrate ingredients separately before you sterilize with a pressure cooker, thus lowering your chances of contamination. I haven't done this but it def won't hurt.

Also, don't keep changing the heat settings while sterilizing with a pressure cooker. This can cause the pressure inside the cooker to change, pushing out water inside the jars. And like shaker said, don't get frustrated. I've had plenty of contam problems so far. I think this is the hardest thing to get a handle of when you first start growing. Good luck