I wanted a lot of sandbags, and did not want to spend any money, so I used paper (bank statements, good paper) fed through a shredder and torn into 2 - 2.5 inch lengths. Roll each of these into a fairly tight roll. When you have a suitable pile it is probably worth taking each one and wiping the end on a Pritt Stick so the end is glued to the roll and will not open out later in the process.
Once you have the required number of rolled tubes you run some PVA long where you want them to go, pick each one up and squeeze it between finger and thumb to flatten it before placing it on the line. Keep the ends in line as these do not look very good. Once you have built up the required number of layers set aside to dry.
Once dry a little filler (I used some pre-mixed Polyfilla) can be added to the visible ends and shaped with a wet knife blade. You may well find there are also holes between some of the bags, add a little filler to fill these in. When all this had dried coat with a thick mix of 'sandbag coloured' paint, I prefer Humbrol dark earth but I happened to have some brown emulsion in a tester pot which was a cheaper option and close enough for my needs.
The consistent size makes up for the lack of detail, even if a few have telescoped out a bit. They can be laid to follow the line of the terrain, I used quite a few to beef up the entrances to bunkers as well as making up some machine gun positions for use at check points and near bridges. This may sound tedious, and it is a bit, but I made all I needed and more during a one hour documentary on the TV.
The example shown below is part of a trench/bunker complex, the ability to follow the curve of the land makes for a more interesting scene.