I think the more relevant question would be “how would we do it today” rather than how did we do it when we were in the same situation you are in.
I would start by trying to figure out what the competition is doing. For delivery, watch them and count deliveries. If sit-down, count tables and turn rate (go to dinner on different nights and take your time). Make some assumptions about average tickets and extrapolate.
I would ask food purveyors how much cheese or flour they would expect to sell to an account like you are proposing and build some assumptions based on product mix and average ticket.
I would go to the city and research the pattern of sale tax receipts for restaurants and mirror that pattern in my month by monthy forecast for the year.
I would take the market rate for leases and multiply the annual rent for competitor’s restaurants by 10 or 12. (If annual rent in your area averages $20 per foot and a competitor has 1500 square feet, I would enter a number of 300-350K in sales (($20X1500)X10=$300,000) and so on) This is NOT valid for any one competitor, but suprisingly accurate for a group of competitors. This might give you “check for reasonablness” on your forecasts (not to mention viability)
Of course, marketing drives business. You will need to make assumptions about marketing before you make assumptions about sales. I would forecast very different numbers for a new store planning to spend 30K in the first year than one planning to spend 10K.