Pretty straight forward, just get an ice cube tray, get some wire and make an appropriate shape to lay down in each mold, then pour plaster inside and wait for it to dry. Whenever you think it's ready, it will be. If it's a little moist you can pop it in a toaster oven for a little while. Sorry for one of my pics being blurry, but I was trying to see how they compare to troop height, and these are pretty decent. I'll be burying them a little bit since I didn't make the bottoms smooth enough, so that will help, and it can help you if your molds are really big.
These aren't done though, I still need to add details like chipped off corners and edges in some parts (hence the wire), and bullet holes and small blasts, and that's where the wire comes in to simulate the rebar inside. Over all, these are pretty good if you want to make terrain impassable for vehicles but usable as cover for troops.