A how-to guide simply on making flat cardboard bases? Oh yes! There are some serious pitfalls and easy wins in this. For most "area terrain" like woods or marsh a base is basically the whole thing anyway
A terrain base must be rigid, light and thin. The best material satisfying these conditions is foamcore used by professional architects. Thin MDF board is a treat for serious projects and there is also a material particularly pleasant for the touch: plywood.
However for most projects simple corrugated cardboard is enough. This is the thing used for most packaging. It is versatile, effectively free and easy to cut. Ikea boxes are particularly useful as they have the right thickness, are quite large and you get them quite clean without any sticky tape or lamination.
You can make the cardboard extra strong by glueing two layers of it in such a way that their corrugations are perpendicular.
To make a base of a cardboard box, you have to:
Personally, I paint all my terrain bases to look like muddy, trampled grass. see how I do it