You buy the fabric, typically you'll double the price when selling to customers. Then you take off additional postage fees/packaging (note the flat fees on postage), payment fees (3-4%), discounts, returns, shopify monthly fees, accounting software etc.
You'll spend about 50% of your day cutting & packing orders, taking them to the post office etc. (or you can employ someone to help) and the rest of the time taking shots for instagram, sourcing new suppliers, working on SEO/blogging and doing admin.
If you're good at marketing you can shift quite a lot of fabric, but it will take you about a year to get your stock levels up to a sustainable amount to pay yourself a wage. If you want a real shop front you will generate most of your profit from running classes as the costs for a high street store are significantly higher than shopify and you gain little in terms of footfall vs online traffic.
This isn't a get rich quick model but over time it works well and is replicable at scale.