Start with the business question, add only the Shopify fields needed to answer it, then apply filters, grouping, sorting, and calculations.
Official answer
A custom report should be designed around the decision it supports rather than by adding every available field.
Begin with the reporting grain, such as one row per order, line item, customer, SKU, or payout. Then select dimensions and metrics that match that grain. Add filters and calculated fields only after the row structure is clear.
This prevents duplicated values and incorrect totals, especially when order-level and line-item-level fields are mixed in the same report.
Follow-up questions
Can I mix order-level and line-item fields in one report?
You can, but first define the row grain. Repeating order-level values on every line item can inflate totals unless calculations account for that structure.
Can a custom report include calculated fields?
Yes. Calculated fields are useful for margins, ratios, conditional classifications, and other metrics that are not stored directly as Shopify fields.