15mm trench lines can be prohibitively expensive for those of us on a budget. Luckily, I figured out how to make trenches out of Sculpey; which is an oven-bake polymer clay. For this tutorial, you'll need the following:
-Ruler (to use as a backing when pressing out trenches, as well as measurement)
-Large and small-handled brushes (for making craters, as well as painting)
First, lay out enough aluminum foil to cover your work surface and accommodate as many trench lines as you plan to make.
Next, take out a hunk of Sculpey and begin kneading it together for a couple of minutes - just enough for it to take on a smoother consistency. When done, begin rolling it out into a long 1" diameter strip. You can make the diameter smaller or larger - this will just make the trench slope shorter or wider, respectively.
Using the ruler, measure out however long or short you want each section to be (mine are 8" for Flames of War). You can reasonably cut each section a little shorter than what you want the final section length to be, as squashing the sections down will make them a little longer. Lay each section out on the aluminum foil as you plan to bake it, as you want to pick the sections up as little as possible from this point until baking.
Now, position the ruler along one long edge of a section - this will now act as a backing while you squash the slope out. Try to keep the ruler at a 90° angle to the section. If you're using a metal ruler, the stamped markings will also make quick woodboard-like indentations in the rear slope (which can later be painted in). Begin on one end of the section and begin squashing the section down, making the opposite long edge "slope" upward into the long edge with the ruler backing. This provides the visual trick of making the troops behind the trench lines look "dug in", without having to alter the tabletop itself. Don't worry about smoothing out the slope. The small indentations and ridges your fingers will make are more realistic. Continue until all sections are sloped.
Again using the ruler, cut down each section to the desired length, again try to keep a 90° angle to the section, so that each section lines up correctly on the tabletop. Corner pieces can be "sculpted" out simply by making sections the same width on all sides as each section and "sloping" two of the four sides.
Once done, you can make bomb craters by using the non-brush end of a brush. Just push it into the slope and work outwards to make a shallow, concave pit. Blast ridges are easily made by making radiating cuts or indentations with a knife or sculpting tool.
Sandbags can be added by rolling out a small diameter of Sculpey, cutting off small pieces, and GENTLY pushing them into the top of the trench slope. Keep a small dish of water close by and dip your finger into it every so often - this keeps the Sculpey from sticking to your finger, rather than the trench.
Once you're done, preheat your oven, stick the sections in and retrieve when finished. You can now paint them as you wish!