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:
1. Lay it flat on the floor
2. Mark the desired shape with a pencil
3. Lift the board (not to damage the floor) and cut the shape out using a sharp knife
4. Stick some masking tape around the edges to hide the corrugation. A painting/masking tape is the thin sticky paper tape used to protect windows' glass and the like from spills of paint. Available in all shapes and sizes from any DIY store
5. Paint the base using an enamel paint
Try to avoid using water-based paints (like acrylic) or water based glue (like PVA) on a cardboard base
— you will warp it. more about warping cardboard and what glue to use
Personally, I paint all my terrain bases to look like muddy, trampled grass. see how I do it