How hard is it to do the "Manual J" A/C tonnage calculation for a
residential detached home?

(Case in point: a 1966 brick ranch home in Denver Metro; 1400 sq ft
main level, 1200 sq ft basement.)

What is the procedure to follow? Download the Manual J spec and follow
the instructions? Or is there some kind of software or Excel
spreadsheet that will make the calc easier?

I know I'm supposed to insist that the HVAC contractor use the Manual
J method, but how do I verify that that they indeed did it correctly,
took all the factors into account, did not cut corners in the
formulas, etc?

cheers