Use a stout piece of lumber, at least 3m long, preferably between 4m and 5m.
Make sure that it is at least 20cm (8") wide, and is fashioned as a square or rectangle.
(Optional) - Attach 4 pieces of lumber to the end of the long piece of lumber opposite to the 1-2m long pice.
The 4 pieces can be as long as you want, and attached in the form of a cross, with one on each side of the square diameter of the pole. This is for stability, and can be used as a stand when braces are attached, or as an anchor when buried deep enough.
Add a brace to the 'tail' of the 'L' (the 1-2 m long part) at about 45º to connect to the main pole.
This helps brace it and prevent it from breaking.
Notch a grove onto 'tail' of the 'L' part, about half a metre from the end.
This is where you will loop the noose.
Dig a hole deep enough and stand up your gallows.
The optional 'cross-bar anchor' while help greatly in providing support for it.
Operation - Your victim stands on a stool/chair, and the noose is looped around the neck. When the time is ready, the stool is kicked away.
Hanging Frame (full-frame-and-stand-with-trapdoor design)
This design is much easier to use, much more efficient to use, and gives the person being hanged somewhat more dignity than the Inverted 'L' design. The complete Hanging Frame design is what collectors are looking for.
Make the floor out of planks.
Ths floor should be around 3m by 2m for a one person gallows.
Make the floor elevated 2-3m by making legs and braces for it, so it resembles a stand or a large table.
Add a staircase for convenience, or you can keep it as is and use a ladder every time.
The Hanging Frame
Take two 3m long posts/poles that are fashioned into square diameters (simliar to that as described in the inverted 'L' design) and attach a third pole 2m long across the top of them both.
You can add two braces from the centrer of the third post attaching to the other two posts at a 45º angle.
Attach the posts to the top of the stand.
If you use longer 5m posts, you can run the posts through the floor of the stand to the ground. Make sure that they are still attached though.
On the floor of the stand, directly underneath the cut notch, cut out a 1m by 1m square.
This will be your trapdoor.
Adding the Trapdoor
Operation - The victim stands on the trapdoor of the stand, and the correct length of rope is run around the notch, tied into a noose, and looped over the persons neck. When the time is right, the lever is pulled, releasing the ratchet, and the persons weight causes the trapdoor to fall.