What you will need..

Small tree (fig, maple, cumquats, pines, are most common)
(I have used a miracle fruit tree, don't use a tree that has large fruit, they won't grow smaller.)

Bonsai Tray

Vermiculite

Soil

Strong Scissors or garden shears.

Bendable wire (copper or aluminimum) Preferably 2 different sizes.
Large one to anchor plant, smaller guage wire for branch training.

Small rocks

Coco Palm Fibre or fly screen

Optional Spagnum Moss & larger rock for placement on top

Step 1: Lets Start With the Bonsai Pot

Cut Coco Palm Fibre or Fly Screen to size that fits the bonsai pot.

Making sure it covers the draining holes.

Then Lightly Cover with small aquarium rocks.

Step 2: Digging Out Your Tree

Dig out the tree.

Wash as much soil from the roots as possible with the hose.

Trim roots, basically anything under 1.2cm in diameter can be cut,
two thirds to three quaters can be removed,
leave mostly large roots basically, but even they can be cut back a bit.

See pictures

Step 3: Anchor Your Plant

Starting at the top of ur plant following the main stem down, wind the wire around the stem and partially around the roots, then poke it through the layer of rocks and coco fibre and hole,
pull wire through until it sits how you want. (straight, lean, even cascade if you like.)

Step 4: Nearly Done

Okay assuming ur anchor is hooked in over the lip of the pot.

Fill with a 50% soil and 50% vermiculite.
Water with a very fine mist, i use a spray bottle.
Twice in summer, else once a day.
Morning sun only for first few weeks, don't bring inside for more than 2-3 days straight.

Place Sandstone rocks (optional)
Place Spagnum Moss on top, also optional.

Wire any other branches if you want them shaped differently.