This is how I've been making my shrubs.
I start with a very simple twisted copper wire frame. You could use fine galvanised tie wire, but the copper tarnishes nicely, so it doesn't look as bad if you miss a bit with the paintbrush.
For shrubs I like to have more than one trunk, so I often bend the wire 'cable' double, then unravel both ends. I don't bend it on the halfway mark because I want uneven heights to the two trunks. I use pliers to pinch it tight at the 'vee', and then it'll be easy to plug it into the landscape.
Here's a pic of the early stages.


DAS clay has been applied and glued, but only where the trunk will perhaps be visible. Pre-painted coconut fibres [hanging basket liner] on hand. The frame has been cut back to a height of 60mm, or 15 ft in OO scale, a bit more in HO. It'll gain some height with foliage, and lose some when plugged in.

Next photo shows a clump of fibre glued in, and more ready to go. Sometimes I twist them tight at one end, as shown here, and it makes for good twig clumps, but that's more important in a specimen tree where the branches will be on show. This one will be a pretty dense shrub, so it doesn't matter really. For smaller shrubs you can do away with the wire and just base them on twisted fibres - quick and easy.



Here's how it ends up - just a tangle of branches. You can cut away any annoying loops in the fibres.



Depending on how it looks with the foliage, I might snip back some of the wires yet.

Next step will be to paint it, but I have to wait for it to dry.

  This morning I finished the shrub.
To start with I cut away any noticeable loops in the fibres, and then teased it all out to a more pleasing shape. There's so much full-strength PVA glue on it that it can take some pretty rough handling, and those fibres are tough!
Then I half-heartedly painted the trunks and branches with a drab grey made from black, red and white. I didn't bother covering it all. Probably not much will be visible anyway.


 Next step is the foliage - in this case Woodland Scenics medium green foliage mat [ #F52] I always test for fit and appearance before gluing the pieces on.
 

 At last it's looking less like something the cat coughed up

More foliage added- just about done.


 Viewed from above:


I keep poking and prodding it as I go to improve the shape.

To finish it off, I randomly dabbed on some glue, then dipped the shrub into some Woodland Scenics coarse turf - burnt grass colour [#T62] That helped make the foliage look a bit older. I also dabbed on some minute touches of orange paint [red oxide + yellow = a fairly tame orange] to hopefully simulate some faint autumn colour.


 Finally a view of it planted on the diorama. [taken with my Olympus] The other photos were taken indoors with my daughter's Fuji and their colour isn't great.


Mike

Thanks for making the duplicate thread here as requested, Mike.

I'm going to lock this thread to preserve its "how to" content. Any discussion and / or questions can be posted in the original post which can be found in Mike's diorama project.