The Golden 1:60 Scale. The best terrain scale for 28 mm miniatures (the D&D standard size and works well with Games Workshop models) is 1:60.

The Golden 1:60 Scale Factor

1 mm  = 60 mm   =>        = 6   cm
1 cm  = 10 mm   =>        = 60  cm
1"    = 25.4 mm => 5   ft = 152 cm
        28 mm   => 5.5 ft = 168 cm

Non-functional Doors: the hole should be around 34 mm tall and 14 mm wide
Functional Doors:     the hole should be around 40 mm tall (model) and 30 mm wide (base)

Dollhouse "quarter scale" (1:48) works relatively well with 1:60.

Remember the scale if aiming for realism. For example, trees are usually modeled way too small; a 1:60 scale adult tree should be >20 cm tall on the board.

Solid cardboard and corrugated cardboard are less optimal; they have warping and strength issues. Perfectly usable otherwise.

If your PVA covered cardboard warps, spread an even layer of PVA to the other side and let dry flat for 24 hours. Warping results from the tension that the shrinking cured glue causes, not the moisture.

Porous materials like wood and foam need to be sealed. Sealing protects the material from warping after shaping and makes paint stick more uniformly. Mod Podge or runny PVA glue works. You can also mix in black or white paint to prime it in one go. Styrene boards have the benefit of never warping from moisture.

I prefer undiluted Mod Podge + a bit of black craft paint and make extra care that I don't introduce any more water to the mix. This is then applied in thin layers.

MDF and plywood are tougher thus slightly harder to work with and might require e.g. a saw, electric drill, rotary tool and such. Fibers in MDF are also toxic so don't inhale any of the residue.

Back panel of most picture frames work as well for small bases. Just make sure you sand the edges well after cutting it to the size.

Tea bags are awesome foliage. Break those bags and see how they look.

6 mm (0.25") thick XPS is strong enough for most uses like walls, floors and roofs.

Even 3 mm can be enough, especially for walls.

This also works for the golden scale 1:60 as 3 mm becomes 18 cm, and modern domestic walls are 10 - 25 cm thick.

Terrain buildings rarely need a separate base if they stand on their own. But if you need a base for your terrain:

• Use over 3 mm / 0.125" thick XPS, fiberboard or plasticard sheets for <30 cm width terrain bases. Most Warhammer terrain is <30 cm.
• Use over 6 mm / 0.25" thick XPS, MDF or plywood for terrain that is >30 cm in a dimension.
• Use over 12 mm / 0.5" thick stuff for terrain that is >60 cm in a dimention.
• It's a good idea to leave at least 1" space around the terrain feature.

Historical wargaming terrain tile tips:

• first figure out the usual playing area
• 60 x 60 cm is common in historical wargaming
• make roads and rivers always exit from the center of an edge
• make road and river exits always the same width

Sources

• Battlefields in Miniature, Paul Davies