Folding small pieces is easy with a little bit of help from your tools.
1. a small pair of straight, smooth jawed pliers or else get the tool in #2...
2. a pair of eyebrow type tweezers with a straight across tip that close flat on the mating edges
3. a craft knife, you can use the sharp or the dull side right against a fold line
If you bend against a straight edge of some type it is easy to fold accurately but the straight edge tweezers are much easier to use on micro sized buildings than using a straight edge.
This is very important to know. Tiny delicate details should be supported by clamping over them with tweezers or pliers if those pieces are right next to a fold. That will often be needed where there is a door or window very close to a fold line, especially on the Micro scales buildings that are made from thin cardstock.
You can also use the straight across tip for folding shorter pieces or reaching across longer sections to get to a fold line.
When I am folding next to an area such as a door or window that is very close to a fold line I clamp over that delicate area to stabilize it putting the tweezers tip against the perforated fold line. Then push on the stronger area on the other side of the bend line to make the fold.
The tweezers are used to clamp over the building side fold line and also go up against door's hinge fold at their tip. Then I slide my craft knife in the cut opening of the door and used the flat of the knife blade to push the door open against the flat of the tweezers tip. This way there is no deformation of the cardstock parts as everything is well supported.