Starting a sourdough starter
Updated: Sep 4, 2020
I should start off by saying that nothing written below is novel - you will find almost all of this information on well-known blogs, like King Arthur Baking, The Perfect Loaf, and FullProofBaking's pages.
My process is most similar to FullProofBaking's, though I differ in my feeding ratios and frequency.

High-level takeaways:
You should have an active starter within 7 days, though it will take another few days to build starter strength.
This starter is built with 30% rye, 70% bread flour. I start with 1:1:1 (30g starter, 30g feed, 30g water) then in the last 2 days move to 1:2:2 if my starter is active (20g:40g:40g). The flour mix you use has a big impact on starter activity and strength, so experiment and see what works for you and your environment. I like rye because it ferments faster than AP and bread flour, so it gives my starter a nice boost. I've used exclusively AP or AP + wheat flour to feed my starter before but those mixes took 14 days to double within 12 hrs.
First 3 days: feed starter once/day in equal parts, with one day without feeding at all. Last 4 days: feed starter twice/day, in equal parts if starter is not consistently doubling yet or higher ratios if starter is active already.
Details:

Day 1:
1. Build your starter mix: 30% rye, 70% bread flour. Fill up a water bottle with filtered water. This is your starter feeding set for the week.
Weigh out 180g rye flour + 420g bread flour, mix well in a container. This is the starter feed mix you'll use for the week.
2. Make your starter: Add 30g starter feed + 30g room temp water to a cup. Cover with paper towel with a rubberband, mark where your starter is so you can track if there is any activity.

Day 2:
Feed starter: Get a clean cup, add 30g of starter. Add 30g starter feed, 30g water. Mark, cover, rest.

Day 3:
Check starter. You should see a bit of activity - bubbles, slight rise. Stir, but do not feed starter today.

Day 4:
You should see more bubbles and your starter should smell a bit fruity.
Feed twice a day today, 10-12 hrs apart: Get a clean cup, add 30g of
starter. Add 30g starter feed, 30g water. Mark, cover, rest.

Day 5:
Feed twice a day today, 10-12 hrs apart: Get a clean cup, add 30g of starter. Add 30g starter feed, 30g water. Mark, cover, rest.
Your starter might double between feedings, but no worries if it doesn't. Trust the process! The picture here is of my starter 8 hrs after its first feeding.

Day 6:
Your starter might have doubled between feedings, but no worries if it didn't. Trust the process!
Feed twice a day today, 10-12 hrs apart: If starter is reliably doubling, get a clean cup, add 20g of starter. 40g starter feed, 40g water. Mark, cover, rest.
If starter is not reliably doubling, continue feeding at 1:1:1 (30g, 30g, 30g)


Day 7:
Your starter might have doubled or tripled between feedings, but no worries if it didn't.
Trust the process! Leave your starter alone for one day (no feedings) to see if it catches any wild yeast. Then, resume twice/day feedings until your starter doubles within 12 hrs.
Feed twice a day today, 10-12 hrs apart: If starter is reliably doubling, get a clean cup, add 20g of starter. 40g starter feed, 40g water. Mark, cover, rest.
If starter is not reliably doubling, continue feeding at 1:1:1 (30g, 30g, 30g)