How do I set up attributes (colour and size) and generate variants?

One shirt in four colours and four sizes is not sixteen products; it is one product with sixteen variants, each with its own price, stock and SKU.

First: the attributes

1. Open Store → Attributes — Figure 1 — and add an attribute such as Colour or Size.
2. Click the attribute to open its values — Figure 2 — and add: white, black, red… or S, M, L, XL.

Second: the variants

1. Open the product from Products → Edit — Figure 3.
2. In the Bulk Generate Variants card pick one or more values per attribute, enter the base price and base stock, then press Generate; every combination is created at once.
3. Adjust the price, compare-at price, stock and SKU per variant, then press Save All.
4. To add a single variant on its own, press Add Variant.

Notes

• The compare-at price must be higher than the price; it is what appears struck through in the store. Leave it empty when there is no discount.
• Each variant holds one value per attribute: “Red + L” is a different variant from “Red + M”.
• Do not delete a variant that appears on past orders; set its stock to zero instead — safer for the record.