Actually the BEST thing is to do the Interior FIRST , (flat on a table ) , then join the panels as usual .... So much easier ...
JMHO, but I think the most critical element of covering is getting an
even coat of glue on the foam and then carefully smoothing /squeegeeing/pressing/stretching the canvas down to the foam
EVERYWHERE , not skipping over any spots. For long walls ( one person) , we stood the canvas up and rolled it out as we were gluing it down ... a workable section at a time ... ( we used blowgun darts as giant stick pins to hold the canvas where we wanted )

Tough job covering by yourself , even a dumb helper is better’n none at all, just somebody to hold the other end ... (I’m not implying anything !!)
