Take a length of copper cored cable. I am making an average sized tree here, so about 10cm. Use thicker cable for taller trees

Remove the insulation, only leaving about 1cm. I leave this on as it is then easier to "plant" when complete.

Use the strands to form the basic tree form. I have found that they do not need to be as complex as you may guess. Obviously bigger trees need more work.

Use a cheap (About £15), small glue gun to coat the wire. This one is 15watts and I use the "low" setting so the glue is not too runny.  It does not need to be perfect as the bumps and grooves add some texture later. Leave some of the end wires uncovered to snag the greenery later..... I do not cover the wire when it gets less than 3  or 4 strands. (I know that these trees do not have the finer twigs and branches, but I am sure that you can live with the disappointment.) It is easier to flatten all the branches before the glue, then twist them back into position.

A coat of basic modelling acrillic paint from a DIY store. I start with a grey base and add a wash of brown when the grey paint is still tacky. Most real tree bark is not brown, but a greyish colour. This gives the trunk and branches a nice texture. Leave to dry.

Add base layer of green. I use Woodland Scenics Polyfibre. Randomly, but loosely.

Spray liberally with cheap hairspray and sprinkle on various scatter, depending on the look you are looking. I cover mostly with a couple of some fine scatter (remembering to do the underside of the tree by holding it upside down) and all sides. I then add some bigger bits to add some interest.

Choose location. Drill hole on base and plant (leaving insiulation on) I find I do not have to glue the trees in allowing for removal for maintanance and transfers

After doing a few you will get the hang of it. I can do one in about 20 mins, but usually make them in batches for 3 or 4. They probably cost about £1 each, including the cost of the glue gun. Building up a few can make a nice scene....