My shed is only 6ftx6ft (space limited) and built about 20 years ago. It was constructed by making 6ft wide frames of 2x2 and well creosoted. Heavy duty polythene was stapled to cover each frame, before nailing on the feather-edge boarding. Each frame had two additional 2x2 uprights, effectively creating three vertical panels a little under 2ft wide. It was skinned on the inside with 1/8" hardboard.
It was later insulated with fibreglass which was available in the UK in "pads", approx 2" thick and approx 2ft wide. I forget the length. This was just a little wider than than the space between the uprights.
The original hardboard skin was removed and the fibreglass pressed in between the uprights. There was sufficient friction to hold the pads in place until the hardboard skin was replaced.
The roof, which was "flat", was 3/4" T&G floorboards supported on 2x2 battens placed at the same intervals as the side members and I was able to do the same here. The roof didn't previously have anything on the inside so 3/8" stirling board was used to support the fibreglass.
The shed is heated with a 300W electric panel heater mounted on the rear wall, controlled by a thermostat on one of the side walls. The thermostat is normally set at about 5 degree C as a "frost" precaution but can be adjusted if I need to work in there, though these days it is used only for storage.
Hardboard is nice and smooth and the whole interior was painted white.
It seems to work well. The shed has no windows as it was originally constructed for use as a darkroom but a ventilation fan was installed in the back wall, also on a separate thermostat, to prevent it becoming a sauna in the summer. Actually, I have found that it can actually be cooler in there in the summer than the outside temperature - presumably because the insulation works both ways - as long as I keep it shut up!
Stuart