First, get your materials together. For this you'll need:
- White glue - I recommend Rona or Home Depot and get the big jug. Not only will it last you the project you're working on, I've had my jug since 2007-ish
- Paintbrush (maybe more than one) - again, the building store. You need a big one, and since it will handle glue, sand and paint it doesn't have to be high end.
- Builder's sand - I bought the 30lb bag years ago and I've still not made a dent in it, despite basing legions of models, making 40 square feet of terrain boards, as well as some miscellaneous hills and other features.
- Newspaper - lots and lots and lots of newspaper. This WILL get messy.
Additionally, have a nice, open space to work on. For me, the gaming table works well, but you could do it on the floor, outside, garage, large table, etc.
Don't forget the caffeine either.
Make sure everything is laid out and you have plenty of room. The newspaper should be double layer at minimum, quadruple layer is better. This will get messy, and there's glue involved. Don't say I didn't warn you.
Next is to take as much white glue as you think you need (it's okay to be conservative, you can always mix up more) and mix it with about one fifth of water. You can play around with it to check the consistency, but it should flow readily from the brush while staying exactly where you want on the workpiece.
It's hard to tell from a still photo but the glue is slightly more runny than you'd get straight out of the bottle. I use an old sour cream or yogurt container to mix in. The advantage is that you can put the lid back on if you have to stop or are doing multiple larger pieces over several days if you don't have the space to lay them out. In this case, it's only a few hills so no problem with keeping extra glue mix.
Now you apply to the surface of the item. This initial layer makes the terrain piece 'sticky' so that when you put the sand on it it doesn't go scattering all over the place. As an aside, I chose not to change the shape of the hills due to a mix of nostalgia, laziness and the fact that stepped hills make for easier model placement during a game.
Here you can see the initial layer of glue:
Now, it's time to add the sand. Just normal, everyday builders' sand. As you can see, the 30lb bag lasts for a long time. I've kept it in an old cat litter bucket for ease of handling.
I normally sprinkle a good amount of sand over the upper surface of whatever I'm working on, like so:
The next step is where the magic happens. Actually, not so much magic, but materials engineering (yeah, I had to throw that in there). Once you've got a good bit of sand on, you get some extra glue mix on the brush and you pat it down so that you have a glue and sand slurry over the object you're making. As needed, add extra sand and pat that down too. You want to have a thorough mixture and a good coating over the entire workpiece.
You can see how the glue and sand forms a mix on the surface. Don't be afraid to adjust the mix. More sand makes for a more rugged surface. Less sand makes the 'dirt' a bit smoother. Make sure, as in the case here, to run your mix down the sides. That way the edges of the hills are thoroughly covered as well. Watch out for brushstrokes in the surface, as that will make it look a bit odd. If you have to move sand around with your brush use a circular motion or move the brush from several directions.
A big benefit of this method is that the terrain pieces will end up with a nice, hard coating of sand and glue mixture. It makes them quite durable, and if you're reconditioning old pieces like I am here, it will pretty much cover any previous decoration. In this case it was a type of modelling grass that was very stiff and scratchy, not as nice as static grass. Once the composite is on, you would never know it had a previous covering.
Once you're done all the pieces you want to do, lay them up somewhere you can let them dry for a while. Sometimes that will be the table or workspace, such as a full terrain board. Normally, I let them dry overnight, that way the glue has lots of time to fully polymerize before you do anything else with it. In this case, I had a shelf that is destined to be moved upstairs, but for now it's empty. I laid out some newspaper, as well as some old blister pack blisters:
The reason to raise the hills above the paper is to keep them from getting glued down. As the next photo shows, gravity always wins so you will tend to have a little bit of pooling at the lowest edge.
To avoid this, I put some blisters from old miniature packaging underneath the centre of each piece.
These have been set to dry overnight. In the case of large items, such as a full terrain board, I've found a small, desktop fan can help. That said, even on 2'x4' boards 24 hours has generally been sufficient. Once that's done, the hills will be ready for painting. However, that's part two...