You don't make it. Tried that back in 1970..
You catch a live culture or you buy a pre-packaged culture. Mine is about 125 years old, caught in Alaska in the 1890s.
It's easy to start and keep.
Day 1:
Place 1/2 cup flour with slightly less than 1/2 cup water in large 1-gallon jar, mix thoroughly, cover lightly (I use paper towel), and let it set in warm place on your counter for 24 hours. Don't place it in direct sunlight.
The dough should be about the consistency of toothpaste. If it's too thin add just a touch more flour and if its too thick add a touch more water.
Day 2:
Feed another 1/2 cup flour and slightly less than 1/2 cup water, mix thoroughly, cover lightly, and let set another 24 hours.
By now you may notice it is getting a little bubbly as the yeast in the flour is beginning to work.
Day 3:
Feed again as before and let it set for 24 hours.
Day 4:
Feed again as before and let it set for 24 hours.
By now you should notice a more bubbly appearance and a distinctive sourdough odor, almost like beer.
Day 5:
Feed again as before and let it set 24 hours. It should now be well soured and ready to use.
After you use some of the dough, cover tightly, and place the remaining dough in a refrigerator to store it. The day before you plan to use it again, take it out of the refrigerator and place it on a counter to warm up to room temp. You may notice that the water has separated some and may even appear discolored. Do not be alarmed it is okay. Just stir it back in. Feed the dough with enough flour-water mixture to match whatever amount you plan to use and let it set overnight to ferment. As it is fermenting, you'll see it rising as the yeast is working. It'll be ready to use the next morning.
It will keep in the refrigerator for weeks but it works best to warm it up and feed it every two to three weeks with a little flour-water mix.
Here's a recipe for sour dough pancakes that I like.
1 egg well beaten.
1-1/2 cups prepared sour dough.
1/4 cup powdered milk.
1/4 cup melted butter or 1/4 cup oil.
1 tsp vanilla
Mix well but do not beat the batter.
Combine
2 Tbs sugar
3/4 tsp salt
1 tsp soda
Add the sugar, salt, and soda about 2 minutes before using. Mix well but do not beat the batter.