Step 1: Design considerations
Also before you start, ensure you have enough branches. I ended up using more than I could have imagined so give your trees a good trimming! Also if you are building next to a fence (as I have done) ensure that you place a polythene sheet between the fence and the soil to avoid rotting of the fence.
Step 2: Digging holes and placing posts
Based on the design that you choose for yourself, dig deep holes at least the height of your garden. You will also need to choose decently thick branches for the posts. I used the biggest I had, which was about 100-150mm diameter. These posts should be reasonably straight with minimal notches and the length should be at least twice the height of the height of the garden (plus some) - 1/2 will be under ground, 1/2 above ground and a little left over to trim flush with the timber beams.
Once the holes are dug, place the post vertical into the hole (you may want to use a level, however this may not matter too much as all natural materials are not perfectly straight). Following this, fill about 3 spades of soil into the hole around the post and then hammer the soil with a piece of flat ended timber. This will make the post sit firm in the ground. If the soil is wet this may be a problem, so perhaps wait until drier weather.
Continue until the level of the hammered soil is about 200mm from the surrounding ground level.
Step 3: Fixing the beams
Once this level is set, cut all of the posts to this string level height.
The next step will be to notch each post so that the beams will be able to sit onto something. Using the level line, calculate how deep the notch will be. This will usually be the size of timber beams you are using. In this example I have used a 100 x 50mm timber section, therefore cut down 100mm and into the post 50mm from the front of the post. Cut out with a saw as level as possible.
Sit the beams on the notches and prepare to fix to the posts.
With the beams in place, drill holes and screw beams to the posts. Ensure that the screw is long enough to go through both. Countersink holes if you like. If you have any corners, these may want to be mitred, however these sort of decisions are up to you.
Step 4: Placing brances
Firstly dig a trench between the posts about 200mm deep and 200mm wide, this will allow the branches to be placed into the earth in order to spread the force between the beams and the ground, meaning the wall will not fall over when soil is loaded behind it. Select only branches which are between 10 and 50mm in diameter and that will span from the bottom of the trench to the top of the beam. Start placing the branches alongside each other, attempting to eliminate or minimise gaps, place branches about 3 deep at this stage. Once the branches have been placed between 2 posts, fill in behind the branches with soil and stamp down with your foot. This will ensure that the branches stay roughly in the same place.
Following the first run with the branches and once they are reasonably firm in their positions, start to trim the branches flush with the top of the timber beams. After a few cuts you will find that some branches are wobbly. If this is the case, you will need to keep squashing in appropriatly sized brances until this wobbling ceases. After a while you will notice that the wall is so dense that no gaps are apparent when looking through the front of the wall. If it is difficult to add new branches, use a hammer to smack the branches into position. Basically, if all of the branches are firm, you will have no structural problems in the future.
By the time all of the branches are cut and each branch does not wobble, the wall should be about 4-5 rows deep - enough to hold soil back from falling through the front.