Warm it up slowly until temperature reaches about 103 F, just before boiling. I made the same mistake as you. The idea is to kill any critter.
Then wait until the temperature drops down to about 80 F, then you add the culture. Otherwise you'll kill the culture.
Mix the culture very well (but VERY SLOWLY).
If you could monitor the temperature with a thermometer, you may adjust the heat power (very very low) to keep the temp at 80 F for the 24 hours period. It will take just a couple of hours to figure out how much power is required.
And, put a lid on the mixture. No air is better to achieve a good yogurt.
I make my own yogurt (2 years of doing that now) and it is very good, and I don't have a yogurt maker.