I've been asked to do a tutorial for making sandbags. To be fair, these are probably the simplest a things you can make from greenstuff, so please don't be offended! However, I will also give a few tips on how to use them on your models.
Step 1. Gather your materials - Greenstuff (though you can use Milliput too but I find it messier) A Stanley knife and a small piece of car body mesh (available from Halfords at about £2 per sheet)

Step2. Roll your GS out to a little under the thickness of a biro. Don't worry about exact sizes!

Step 3. Using the BACK of the Stanley blade, section off the roll into suitable lengths.

Step 4. Seperate your bags. By pushing down with the back of the blade, you will find it has already created a "flour sack" appearance. Tidy the ends. If you get this tight, it will look like a seamed end to the sack, but don't worry too much about it.

Step 5. It is best to put the bags in place first. Once in place, gently roll the body mesh over the bag with your finger. A slight pressure will flatten the bag too.

See, that was easy!!
Good uses for sandbags1) Sandbag emplacements
2) Representing extra armour on vehicles
3) Hiding accidental holes in plastic or resin on vehicles
4) Use it to "join up" items of stowage to make them look like they should be there as opposed to just stuck on the vehicle.
5) Use them to prop up easilly broken parts such as open resin hatch covers.
As you can see from this photo, I have used them here to cater for 2, 4 & 5.

Here I've used them to represent a fortified building

Just one last tip from me. I find that they look a little better if they are not applied in a uniform way. Admitedly this is not to everyones taste, but I often have one or two bags laying the wrong way in a wall for example.
The most important thing to remember is to make the bag mould itself to the contours of your model, or to the last layre of bags. Do NOT allow light to be seen under them. This often happens when people use the plastic bags that can be bought. It really does look unrealistic then!
Go forth and populate your models with sandbags!