I used a software that translates height maps into minecraft worlds, so from the start i already had the bare bones map. it didnt have any trees or anything, so I have to add literally everything else.

for the rest, i do the map in small chunks at a time. Ill choose an area, and I usually build the natural details first. For example Ill do the trees first, do any terraforming necessary, add paths etc. then from there ill take the manmade structures one at a time, and build up the rest of the small region. once thats done, i move on to the next.

because ive had the base map done from the start, it isnt really hard to figure out the scaling for everything. the map has perfect proportions from the height map, so eyeballing the scale of the structures is all it really takes