Four decisions before writing code: who uses it, the first flow, which systems it must connect to, and the scope of the first release.
A website or app slips when the scope is still vague. These four questions are enough to start in the right direction.
Who uses it
Ordering customers, internal staff, or both. Each group needs different screens and permissions.
The first flow
Pick one job the first release must finish, for example submitting a wholesale order. Leave the rest until that flow runs reliably.
Which systems it must reach
List the warehouse, payment, invoicing or CRM an order has to pass through. Without that list, the new software sits alone and staff retype the numbers.
What done means
Write down what the first release does and what it deliberately leaves out. That boundary keeps the quote and the schedule tied to the work that must ship.