Fit the underlay and carpet first. You will need spray contact adhesive to do that and you don't want it all over the dash.
It is much easier to fit the carpet to the footwells and tunnel with increased access without the dashboard and steering column.
I am just fitting dash #5. Check everything is connected properly before bolting in place. Look at all the gauges, pump, flasher, voltage stabiliser etc to make sure all the connections are in place, oil pipe tight, speedo cable tight, etc as it is a nightmare to do anything when the dash is fitted. You have to do everything upside down, with minimum space, a torch and it is very uncomfortable like that.