Materials

Rubberised Horsehair

Twig. It is surprisingly difficult to find a suitable twig. It should have as many small branches as possible, as well as not being too fragile. Shrubs such as azaleas are quite good, but can be a bit on the fragile side.

Foam flock

Arid style base

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PVA

Dark brown emulsion paint (or acrylic if you feel extravagant).

Trunk & base

Cut base to shape
I usually use mounting board, because it gives a stronger join when the trunk is stuck into it.

Weight base
To stop the tree falling over so often, it is best to weight the base down. The ideal way to do this is a metal washer with a large enough hole in the middle to fit the trunk through, as this will allow you to stick the trunk down with the stronger 'embedded' method. Alternatively use a coin (I generally use the UK 2p - which is cheaper than many washers). The disadvantage of this is that you will have to stick the trunk on top, which isn't

Stick tree to base (the embedded method)
Trees need to be stuck down at this stage, using plenty of PVA (or epoxy resin). To ensure that trees are well stuck to mounting board, cut a rough hole in it with a scalpel which is slightly smaller than the trunk. Put PVA around the hole and force the trunk in. This will create a much stronger junction than just sticking it on top.When the glue has set use a scalpel to trim the trunk flush with the bottom of the base.

Stick tree to base (the 'on-top' method)
If you are using a coin in the middle of the base to weight it, then you won't be able to embed the trunk. Instead you will have to stick it on top. Don't use anything other than epoxy resin for this, including a glue gun. To get a strong bond, make a grid of fine scratches on top of the coin.

Reinforce trunk
You can strengthen the bond by packing epoxy putty around the base of the trunk, but this shouldn't be necessary if it has been properly glued.

Texture base
One option is the arid basing method described here, but any sort of sand/flock type arrangement is suitable as the trees are fairly generic.

Foliage

There are two options for foliage:

Rubberised Horsehair
This is a difficult substance to find, but ideal for foliage which comes as an inch thick sheet. Tear chunks off, or cut them with a good pair of scissors. Then tease the clumps out slightly to give it a wilder look. Put a few large blobs of PVA on the twig and then glue the horsehair to them. (If you spread the glue too thinly, you won't get very good contact).

It is a yellowy brown colour which is almost useable untouched, but this effect can be improved by dabbing lots of watered down dark brown emulsion paint to the tops and sides of the horsehair, before sprinkling them with flock.

Steel Wool
The disadvantages that steel wool has compared to horsehair are:

It doesn't have the stiffness that horsehair has, you can drape it over twigs, but you can't easily make a free standing bush out of it.

It has to be painted, and painting it is very difficult, the paint tends to come off, leaving obvious shiny areas.

It doesn't have the messy 'bushiness' of horsehair

Having said all that, it has been used to excellent effect in the example above, and these larger trees. It can look good, but it takes more work. To paint it, you will need to apply several coats of brown spray paint. Then paint it with PVA and flock it liberally.