Fold up chasses allow you to easily and correctly align the sideframes, with just a couple of careful bends.  After a quick check you can then solder in a few spacers to form a strong, rigid structure, ready to take further components.  The picture shows the essential chassis pieces from a Worsley Works kit (14.2mm version) for a Great North of Scotland Class V locomotive.





Some fold up chasses include spacers as fold up pieces within the main chassis fret, which makes things even easier. This example is from the Brynkits GWR 2251 chassis kit (12mm version).



One alternative to the fold up approach is the slot-and-tab chassis.  Tabs on the sides of spacers fit into matching slots in the sideframes, which helps align  things correctly.  This approach requires a bit more care, to ensure the spacers are properly bedded, and a bit more soldering, but is still quite easy.  Unlike fold up chasses, you don't normally need different kits for different gauges; the different width is catered for by different width spacers, as in this fret from the Society Modelex-produced GWR 45XX chassis kit.