There are a couple of options for sterilisation of potting soil by heat treatment.

1. Solarisation: Spread out soil in a thin layer and moisten. Cover with heavy clear plastic, anchor edges and pull tight. Leave for a month (at least). Requires plenty of sun.
2. Steam sterilisation: Use specifically designed equipment, or heat soil in a suitable container over a fire. Adding water will create steam and soil needs turning so all parts get exposed to even heating.

Obviously, you want to treat the smallest volume of soil possible. Is the source of rye grass sprouts likely to be the yard waste compost? Most weed seeds are destroyed at 77 degrees Celsius for 30 minutes in moist conditions. So hot composting to 60 degrees Celsius for a week would achieve this. An internet search on ‘potting mix’ will return many sites with all kinds of recipes.