"How to make Wargames Terrain" is now an old, out of print publication from Games Workshop. That said, if you can get hold of it, it is an absolutely terrific resource for beginning and moderate experience terrain builders. I got mine through a GW raffle many years ago, and it has been a source of inspiration and an excellent reference manual ever since.
The second image (below) gives a short table of contents of the book and gives a great feeling for what is included in the book. It starts off by suggesting the types of project that one might undertake, ranging from complete epic board building through to something a bit more modular or limited in scope (singular buildings, fences, etc.).
The first section of creating a gaming board is a solid opener - with many ideas for the beginner to think about and possibly attempt. Do you want a modular Cities of Death board, or something like the rolling hills of the Rhovanion? How many subsections will be in your board - and how will you store it!?
The section on hills is straight forward and easy enough for the beginner to tackle a few projects. But as we progress in, some of the innovations get more complex. The woodlands do require some effort (from personal experience of building trees) and the alien trees will certainly require more time than basic ones.
I have personally always found water features hard to pull off in a convincing manner - there's something about flowing water that always trips me up in its execution to model. But the book demonstrates a number of ways to do this, as well as the more basic routine of how to create lakes / ponds / rivers, etc. Roads are death with too in a comprehensive manner, and this is followed up with many ideas for obstacles - plenty there for the beginner as well as more experienced terrain modeller.
The buildings section progresses on and showcases a number of great looking buildings, but many of them also look tough to execute. The book is rounded off with basing, and terrain boards. The terrain boards are inspiring, but the very last section then goes and posts a good number of pictures that just whet my appetite to try to build something even more complex and themed: big boards with all the kind of terrain that go with them … if only I had the time in real life! As more of an appendix, the book terminates with a tour of the regular materials (styrene, flock) and less regular materials (brush bristles, sponges) that might be used.
Overall, this is a fantastic addition to a serious modellers library who are starting out, or have a bit more experience under their belt and are looking to expand their repertoire.