My mother used to do it in the oven, but it smelled bad. Why not just buy a bag of sterile seed starting soilless mix?

Well I remember my DH coming home from work and me in the garden and the dirt cooking in the oven....and him coming out to the yard and asking....What is THAT for dinner????
Buy some....easier and smells better!
BUT....if you spread some soil out in a shallow pan in the hot sun with a black plastic bag over it....it will pretty well kill all the weed seeds....
Linda C

I would think you only need to sterilize the top few inches to prevent seeds sprouting.

You could microwave or bake enough soil to fill the top third of your pot and just use regular soil for the bottom. I top my pots with some type of mulch (chopped leaves, fine bark, sheet moss) to keep blown in seed from sprouting and retain moisture. Good luck.

Dowse it well with boiling water. Sow or plant after it cools.