Making yogurt

Yes — that is basically the right traditional controlled-temperature method, with a couple of important adjustments for 1.5 gallons.

For your 1.5 gallons of whole milk

1. Heat milk to 180°F (82°C).

   • Stir periodically so it doesn’t scorch.

   • Hold around 180°F for 5–10 minutes if you want a thicker, more reliable yogurt. This denatures whey proteins and improves the set.

2. Cool to about 110–115°F (43–46°C).

   • Don’t add the starter while the milk is still near 180°F; that can kill the cultures.

3. Add your starter.

   • About 1 cup of plain commercial yogurt is reasonable for 1.5 gallons, provided it contains live active cultures and isn’t heavily sweetened or flavored.

   • I would actually use ½–1 cup, rather than assuming more starter makes better yogurt. Too much starter isn’t necessarily better.

4. Mix gently but thoroughly.

   • A good technique is to take some of the cooled milk into a bowl, mix the yogurt starter into that first, then stir that mixture back into the large pot.

5. Pour into clean glass jars.

   • Cover the jars. They don’t need to be hermetically sealed; loose lids are fine.

6. Incubate around 105–115°F for about 6–12 hours.

   • Since you like a fairly traditional yogurt, I’d start checking around 7–8 hours.

   • Longer = generally more tangy and firmer, but eventually the yogurt can become excessively sour and separate.

7. Refrigerate immediately once it has set.

   • Don’t stir it before refrigeration if you want a firm set.

   • Chill for several hours before eating; it will firm further.

One thing I’d change from your plan

Don’t simply put the jars on the counter overnight unless your room is reliably around 105–115°F—ordinary room temperature is too cool and inconsistent for this method. You need an incubation method that maintains roughly that temperature: yogurt maker, Instant Pot yogurt setting, oven with a suitable low-temperature setting, insulated cooler, etc.

And because you’re using 1.5 gallons, a yogurt maker designed for small batches may not be convenient. An insulated container or large pot with controlled warm-water incubation can work very well.