This is actually my first instructable, and it is also the first time I've done a diorama, but I've been wanting to do this for a long time.
But I had a hard time finding any suggestions for home made alternatives for static grass / miniature fake grass, so I think thats also worth mentioning.

Hope you like it! I enjoyed making it, and I think this would be a great afternoon project for kids as well.

You will need:

A flat plastic container. In my case I used an empty case for a cassette tape
Sand
Different kinds of grass looking dried herbs (I used oregano, thyme and especially ramsons worked really well. I think dill would also work nicely)
A tree looking twig (You might want to pick it from a bush, since the individual twigs are more dense than a twig from an actual tree)
Glue (something that dries up transparent - NOT superglue!)
Small stones and pebbles
An interesting centerpiece, maybe a small toy house or animal. I used a tiny cow
Paint (Optional, if you're not that great a painter or want to leave it transparent)
Thin metal wire/gardening wire, and something to cut it with

Step 1: Modifying the Tree

It is very unlikely that you will find a twig that fits perfectly, and has the right amount of density or length in the individual twigs.
But with a little inspiration from Dr. Frankenstein, you can modify it to be the right length, and have all the twigs in the right direction (since it will have to be somewhat flat, and still nice looking from the side)

Cut out a middle piece of the twig you want to shorten, and throw that middle piece out. Be careful not to cut it somewhere where the twig goes from thin to thick within just a few millimeters, since it will look odd when you glue those two parts together due to thickness differences.

Cut a little piece of wire, also just a few millimeters. Its better to cut off excess length, rather than have it too short. Even it out as much as possible!

Push it in to the soft middle in one of the twigs, and dip it in glue. Push the other end in the the other twigs soft middle. Whipe off any excess glue.
Now its shorter, and you can also rotate it a bit, if necessary.

Trim it even more, to make it fit the size of the container. You might also want to test it, and put it in the container, to make sure all the twigs are the right length and in the right directions.

Now set it aside, and let the glue dry.

Step 2: Make a Background

If you're using a cassette tape case like me, don't forget to remove those thingies thats meant to hold the tape in place. They can easily be removed with a wire cutter.

If you're not that confident with painting, or just want it transparent you can skip this part. Alternatively, you can also go find a nice picture of a landscape scenery on google, scale it to the right size, print it and glue it on the back.

But if you want to paint, here are some tips:

Paint the sky first. Its better to work with layers, and make the layer furthest away first, rather than work around layers.
A nice blue summer sky often has a gradient, and is lighter (sometimes almost white) near the horizon. I just used two shades of blue, and white to make this gradient. You might want to do more than just one layer of paint.

I also recommend painting the clouds while the blue is still a bit wet. It makes the shadows of the clouds blend in more natural, rather than if you used grey for shadowing.

Let this dry before you do the next layer, and details.

Step 3: Adding Leaves

While the paint is drying, your tree has probably also set. Now its time to add some leaves.

I found that ramsons was the best for this job. You can use whatever herb you think look most like tiny leaves.

Dip each end of the twigs in glue (but only take 2-3 at a time, its easier to work with, rather than have all the twigs sticky at the same time). Make it a nice thick drop. And if you get strings from the glue, see if you can work it to your advantage somehow.

Dip or sprinkle the herb on the glue.
To make the leaves look a little more dense, also put a few drops on glue in the middle of the 'tree', rather and only at the end of the twigs.

Once its dry, you can paint some tiny spots of red, lilac, blue or whatever you like for flowers. Or don't, if its a flowerless tree. I just felt that a little colour really spiced up the scenery.

Step 4: Finish the Background

Once the sky is dry, you can move on to the next layer. I made a couple of hills with a little forest.
I also figured I might as well use the shapes of the thingies that used to hold the tape, and painted them as windmills. It reminded me of where I grew up as a child (I counted 24 of those, within eyes reach, before I moved out..).
You can maybe paint them as clouds, or just cover them up any other way you like.

Don't forget to also paint the background of the lid. Even if you might fill sand all the way, you never know. And it looks weird if the last part is transparent, and the rest is not.
Also try to get the ground colour as close to your herbs colours as possible.

Before you do anything more with it, either let it dry completely, or take off the lid from the rest, so you don't get any sand or herbs on your wet paint.

Step 5: Making the Sand

Mix glue and sand. And by glue, I mean LOTS of glue! I used contact glue, but in hindsight, I think school glue might have been better, as long as it dries up transparent.

Mix it well together, until you got a very sticky muddy pile of sand. Now you have to work quickly!
Put it in the bottom of the lid, and work it down with your fingers or maybe a thick stick. If you want, you can add some tiny rocks that can be viewed from the side (or maybe paint a little skeleton before this, that could be really cool!).

Put the tree in place and the rocks. The rocks most likely wont hold for now, but press them into place anyway, to make an impression in the sand.

Step 6: Add Some More Glue!

Once the sand have firmed up a bit, remove the stones again. If your tree is also very wobbly and can be removed too easily, take that out as well. But try to keep the whole as intact as possible.

Add some glue and even it out with a little stick (maybe an extra piece of twig from the trimming). Put the stones (and tree) back in place, and sprinkle some of your 'grass' herb. Make sure to get into every little crack around the rocks and into corners.

Step 7: The Final Piece!

I didn't trust just glue in this case, so I added some 'stilts' for my cow. Some wire in each leg, evened out as much as possible.
My cow is made from silicone/rubber of some kind, so this as fairly easy. If your animal/item is made from something harder, obviously this isn't possible. Then you just have to rely on glue.
The cows wire stilts were carefully pushed into the sand.

I also wanted a bit of variation in my landscape, so I picked a few leaves from some young fennel sprouts I had (could just as well have been dill really).
To make them stay in place, I made a little ground spike of sorts.

Now the diorama is done! Enjoy it by putting it on a shelf, or maybe hang it on the wall.