Start this process in the morning. Combine 1 cup of spelt flour with ½ cup of lukewarm water in a large food holding container that you won't need for a few days. I used a casserole dish. Cover the container with a paper towel or clean dish towel and leave it at room temperature (70°F) for 24 hours. Don't use an airtight lid on your container.
My house tends to be a little chillier than 70°F at this time of the year, so I nestled the casserole dish up to a heating pad set on low. This is a good way to keep your starter warm enough if you don't want to crank up the heat in the house.
The next morning (day 2), measure out ½ cup of your starter and discard the rest. (Pro tip: do not miss my article above for ideas to use your starter discards!) Mix the reserved starter with 1 cup of spelt flour with ½ cup of lukewarm water. Cover your holding dish again with a towel and let it rest at approximately 70°F for another 24 hours.
On the third morning, check your starter. It should be bubbly and have that characteristic sourdough smell. Starting today, you are going to begin feeding your starter twice per day (12 hours apart), once in the morning and once at night.
For each feeding, you are going to mix ½ cup of the starter with 1 cup of flour and ½ cup of lukewarm water. Then you should cover the holding container loosely and let it rest at 70°F until the next feeding. Repeat these morning and evening feedings on days four and five.
You are going to end up with starter to discard with each feeding since you will always start with more than the ½ cup you need. It can add up to a lot of food waste if you don't use it!
On the morning of day six, check your starter again. It should be nice and bubbly, have doubled in volume, and smell like sourdough. If it is not as active as described, continue feeding it for another couple of days before continuing on. When the starter looks and smells like it is ready, give it one more feeding of 1 cup of flour with ½ cup of water. Do not discard any starter this time! Let it rest at 70°F for 6 more hours (this will be the evening of day six if it did not need extra feedings).
After the 6-hour rest, measure out and set aside 1½ cups of your active starter. This is what you'll need to make bread now (instructions below). Put an additional ½ cup of starter in a glass canning jar or crock; you'll be keeping this long-term for when you want to make more bread. Any additional starter that you have can be discarded.
Feed the ½ cup of reserved starter 1 cup of flour and ½ cup of lukewarm water. Let it sit at room temperature for 3-4 hours until it gets nice and bubbly, and then pop it in the fridge. Don't cover the canning jar or crock tightly.