I am experimenting with using my 6mm terrain with 15mm figures on a DBA/HOTT sized table. The trouble is I’ve only got a small collection of 6mm terrain and this is from old Military Miniatures range crafted by Mark Strachan. I like the terrain, and the paint job by Gordon Roach, but Military Miniatures morphed into BattleFront and they no longer sell this stuff. So I decided to supplement my collection by making my own.
The photo shows a homemade stream section to a commercial one. You can see differences between them in the photo but on the table they aren’t noticeable.
Home made and painted stream section (left) versus commercial (right)

I describe the method below. Mine are to match existing 6mm pieces but the same process applies for other scales.
Method
Step 1. Cut MDF to shape
Firstly I cut out some shapes from 2mm MDF. I made the stream sections roughly 18mm wide. Long sections were 150mm in length but I made some variations (see below). I bevelled the edges with a craft knife.

Step 2. Shape stream banks
Next I built up the banks with mlliput. You could also use Green Stuff. Both have the advantage that they set rigid.

Step 3. Glue Sand to Banks
A bit of diluted PVA glue and some sand went onto the banks of the stream.

Step 4. Paint
Lastly I painted. I use the Black Undercoat Method so started with a black base layer. The water has a base coat of Coat D’arms 207 Dark Blue. I then painted patches of lighter shades the same 207 Dark blue mixed with white; then I repeated that with even smaller patches of an even whiter shade. The banks were painted with Coat D’arms 230 Camouflage Green.

Step 5. Varnish
I sprayed the painted sections with a matt varnish. Then I painted the water part of the feature with a gloss varnish.
Shapes
I did a variety of shapes including a few more or less straight sections, two Y sections, two short straight sections, some short curves and a ford.

Bridges
I got some stand alone bridges from Timecast (37/009-6mm scale Rural Stone Bridges). These are very nice but need building up to butt up against my roads and streams which are based on 2mm MDF. So I put the bridges on 2mm MDF as well. I also had to narrow the stream to match mine (the Timecast rivers are much wider).
Here is the Timecast bridge modified to fit home made streams and roads.

The stone of the bridge is painted in Coat D’arms 221 Horse Tone Dun then has a dry brush of Vallejo 007 Pale Sand. The road, including that part of the road crossing the bridge, is painted in the same way as the earth style I use for flocking.