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 but the ability to follow the curve of the land makes for a more interesting scene.. Once you have built up the required number of layers set aside to dry.
For the early sets I used a little pre-mixed Polyfilla on the visible ends and in any visible holes between the bags, shaped with a wet knife blade. More recently I have been using acrylic sealant (£1 a tube at my local 'pound shop'), this can be painted on with a wet brush and the brush can be washed in water before the sealant sets to clean it. It has to be acrylic sealant not silicone as siicone does not take paint. You can mix a little paint with the sealant in a paper cup to pre-colour it but it is easier to just paint it once the whole thing has set. Sandbags come in a range of shades, some were a pale olive green others were lighter (even in Europe) so colouring is not an issue. I have used Humbrol dark earth but light brown emulsion in a tester pot is a cheaper option.
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 and provide protection for forward stores dumps 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 rolled all the sandbags I needed and more during a one hour documentary on the TV.