Here is my "how to, in a month" from starting to build to 99% completed. 
In January 2024, a Pal and I organised a large Pelennor Fields battle which took five players at my local club in Wales The Carmarthen Old Guard, based in Five Roads, near Llanelli.
Link: (1) Carmarthen Old Guard (C.O.G) Wargaming, RPG Tabletop & Board Gaming Club | Facebook
Down the pub after the game, Dan mentioned he had a couple of the Osgiliath buildings and what a great game that would be fighting over the ruins. I said OK. I have some buildings half built, let's do it in February and we got cracking.
I had made two buildings which I purchased from ebay and another club member kindly donated four more unmade ones. I had a couple of Age of Sigmar ruins and I bought some extra bits from ebay:
First Job was wash all the buildings in soapy water, then undercoat an Earth Brown Aerosol, then paint all the woodwork and floors a medium grey. I use emulsion paints when painting scenery, as it is more hard wearing than model paints and does not require varnishing.
I also dabbed large areas of the grey on the walls, to give depth when dry brushed. here you can see the grey "bleed through" the white.
After the white dry brush, they all got based on 4mm plastic. 
Next job, PVA (straight) and glue wallpaper (floor slates), super glue any big bricks, used old 20mm plastic bases for pavement slabs (snipped and scored for damage)
Then PVA 50/50 water mix then on with the builders Sand
Some of the pieces needed ramparts, so coffee stirrers and some left over plaster bricks were on the agenda, then a coat of watered down Chocolate brown emulsion (avoid painting the wallpaper floors), when dried overnight, dry brushed white & greys.
