We used a combination of the torn out pages of a big scale road atlas and google maps on my phone. We tended in the end just to plan day by day, fairly often within the day. Most days required very little in the way of directions anyway and when there were a lot of junctions we tended to be on google maps.
Using Google’s “on foot” route planning option for shorter sections can be a good way to pick nice direct routes on small quiet roads. Poor mobile reception was no problem because we had a paper map backup, but in the end I don’t remember it coming to that.
We reckoned our route was pretty much exactly 1000 miles, via Arran, keeping off A roads as much as possible, and with a couple of small detours to stay with family/friends en route.